Implemented four eyes principle.
diff --git a/api/src/main/java/io/mifos/portfolio/api/v1/client/PortfolioManager.java b/api/src/main/java/io/mifos/portfolio/api/v1/client/PortfolioManager.java
index 496bd2d..beea82f 100644
--- a/api/src/main/java/io/mifos/portfolio/api/v1/client/PortfolioManager.java
+++ b/api/src/main/java/io/mifos/portfolio/api/v1/client/PortfolioManager.java
@@ -329,6 +329,7 @@
           produces = MediaType.ALL_VALUE,
           consumes = MediaType.APPLICATION_JSON_VALUE
   )
+  @ThrowsException(status = HttpStatus.CONFLICT, exception = TaskExecutionBySameUserAsCaseCreation.class)
   void markTaskExecution(@PathVariable("productidentifier") final String productIdentifier,
                          @PathVariable("caseidentifier") final String caseIdentifier,
                          @PathVariable("taskidentifier") final String taskIdentifier,
diff --git a/api/src/main/java/io/mifos/portfolio/api/v1/client/TaskExecutionBySameUserAsCaseCreation.java b/api/src/main/java/io/mifos/portfolio/api/v1/client/TaskExecutionBySameUserAsCaseCreation.java
new file mode 100644
index 0000000..f4de44f
--- /dev/null
+++ b/api/src/main/java/io/mifos/portfolio/api/v1/client/TaskExecutionBySameUserAsCaseCreation.java
@@ -0,0 +1,22 @@
+/*
+ * Copyright 2017 The Mifos Initiative.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.mifos.portfolio.api.v1.client;
+
+/**
+ * @author Myrle Krantz
+ */
+public class TaskExecutionBySameUserAsCaseCreation extends RuntimeException {
+}
diff --git a/component-test/src/main/java/io/mifos/portfolio/AbstractPortfolioTest.java b/component-test/src/main/java/io/mifos/portfolio/AbstractPortfolioTest.java
index efca4f2..9e07c3b 100644
--- a/component-test/src/main/java/io/mifos/portfolio/AbstractPortfolioTest.java
+++ b/component-test/src/main/java/io/mifos/portfolio/AbstractPortfolioTest.java
@@ -270,7 +270,7 @@
     ret.setName("feep");
     ret.setMandatory(false);
     ret.setActions(new HashSet<>());
-    ret.setFourEyes(true);
+    ret.setFourEyes(false);
     return ret;
   }
 
diff --git a/component-test/src/main/java/io/mifos/portfolio/TestTaskInstances.java b/component-test/src/main/java/io/mifos/portfolio/TestTaskInstances.java
index 5f3d324..f5e446c 100644
--- a/component-test/src/main/java/io/mifos/portfolio/TestTaskInstances.java
+++ b/component-test/src/main/java/io/mifos/portfolio/TestTaskInstances.java
@@ -15,13 +15,16 @@
  */
 package io.mifos.portfolio;
 
+import io.mifos.core.api.context.AutoUserContext;
 import io.mifos.core.api.util.NotFoundException;
 import io.mifos.core.test.domain.TimeStampChecker;
+import io.mifos.portfolio.api.v1.client.TaskExecutionBySameUserAsCaseCreation;
 import io.mifos.portfolio.api.v1.domain.Case;
 import io.mifos.portfolio.api.v1.domain.Product;
 import io.mifos.portfolio.api.v1.domain.TaskDefinition;
 import io.mifos.portfolio.api.v1.domain.TaskInstance;
 import io.mifos.portfolio.api.v1.events.EventConstants;
+import io.mifos.portfolio.api.v1.events.TaskDefinitionEvent;
 import io.mifos.portfolio.api.v1.events.TaskInstanceEvent;
 import org.junit.Assert;
 import org.junit.Test;
@@ -93,6 +96,32 @@
     Assert.assertTrue(allTasksForCase.contains(executedTaskInstance));
   }
 
+  @Test
+  public void shouldMarkBasicTaskNotExecuted() throws InterruptedException {
+    final Product product = createProduct();
+    final TaskDefinition taskDefinition = createTaskDefinition(product);
+
+    enableProduct(product);
+
+    final Case customerCase = createCase(product.getIdentifier());
+
+    portfolioManager.markTaskExecution(product.getIdentifier(), customerCase.getIdentifier(),  taskDefinition.getIdentifier(), true);
+    Assert.assertTrue(eventRecorder.wait(EventConstants.PUT_TASK_INSTANCE_EXECUTION, new TaskInstanceEvent(product.getIdentifier(), customerCase.getIdentifier(), taskDefinition.getIdentifier())));
+
+    final TaskInstance executedTaskInstance = portfolioManager.getTaskForCase(product.getIdentifier(), customerCase.getIdentifier(), taskDefinition.getIdentifier());
+    Assert.assertNotNull(executedTaskInstance.getExecutedBy());
+    Assert.assertNotNull(executedTaskInstance.getExecutedOn());
+
+    portfolioManager.markTaskExecution(product.getIdentifier(), customerCase.getIdentifier(),  taskDefinition.getIdentifier(), false);
+    Assert.assertTrue(eventRecorder.wait(EventConstants.PUT_TASK_INSTANCE_EXECUTION, new TaskInstanceEvent(product.getIdentifier(), customerCase.getIdentifier(), taskDefinition.getIdentifier())));
+
+    final TaskInstance unExecutedTaskInstance = portfolioManager.getTaskForCase(product.getIdentifier(), customerCase.getIdentifier(), taskDefinition.getIdentifier());
+    Assert.assertNull(unExecutedTaskInstance.getExecutedOn());
+    Assert.assertNull(unExecutedTaskInstance.getExecutedBy());
+
+    final List<TaskInstance> tasksForCaseExcludingExecuted = portfolioManager.getAllTasksForCase(product.getIdentifier(), customerCase.getIdentifier(), false);
+    Assert.assertTrue(tasksForCaseExcludingExecuted.contains(unExecutedTaskInstance));
+  }
 
   @Test(expected = NotFoundException.class)
   public void shouldFailToMarkNonExistantTask() throws InterruptedException {
@@ -105,4 +134,40 @@
 
     portfolioManager.markTaskExecution(product.getIdentifier(), customerCase.getIdentifier(),  "blubble", true);
   }
+
+
+  @Test(expected = TaskExecutionBySameUserAsCaseCreation.class)
+  public void fourEyesCannotBeMarkedByCaseCreator() throws InterruptedException {
+    final Product product = createProduct();
+
+    final TaskDefinition taskDefinition = createTaskDefinition(product);
+    taskDefinition.setFourEyes(true);
+    portfolioManager.changeTaskDefinition(product.getIdentifier(), taskDefinition.getIdentifier(), taskDefinition);
+    eventRecorder.wait(EventConstants.PUT_TASK_DEFINITION, new TaskDefinitionEvent(product.getIdentifier(), taskDefinition.getIdentifier()));
+
+    enableProduct(product);
+
+    final Case customerCase = createCase(product.getIdentifier());
+
+    portfolioManager.markTaskExecution(product.getIdentifier(), customerCase.getIdentifier(),  taskDefinition.getIdentifier(), true);
+  }
+
+
+  @Test
+  public void fourEyesCanBeMarkedByDifferentUser() throws InterruptedException {
+    final Product product = createProduct();
+
+    final TaskDefinition taskDefinition = createTaskDefinition(product);
+    taskDefinition.setFourEyes(true);
+    portfolioManager.changeTaskDefinition(product.getIdentifier(), taskDefinition.getIdentifier(), taskDefinition);
+    eventRecorder.wait(EventConstants.PUT_TASK_DEFINITION, new TaskDefinitionEvent(product.getIdentifier(), taskDefinition.getIdentifier()));
+
+    enableProduct(product);
+
+    final Case customerCase = createCase(product.getIdentifier());
+
+    try (final AutoUserContext ignored = this.tenantApplicationSecurityEnvironment.createAutoUserContext("fred")) {
+      portfolioManager.markTaskExecution(product.getIdentifier(), customerCase.getIdentifier(), taskDefinition.getIdentifier(), true);
+    }
+  }
 }
diff --git a/service/src/main/java/io/mifos/portfolio/service/internal/command/handler/TaskInstanceCommandHandler.java b/service/src/main/java/io/mifos/portfolio/service/internal/command/handler/TaskInstanceCommandHandler.java
index 3d28ae7..5e32df0 100644
--- a/service/src/main/java/io/mifos/portfolio/service/internal/command/handler/TaskInstanceCommandHandler.java
+++ b/service/src/main/java/io/mifos/portfolio/service/internal/command/handler/TaskInstanceCommandHandler.java
@@ -82,8 +82,14 @@
         .orElseThrow(() -> ServiceException.notFound("Task instance ''{0}.{1}.{2}'' not found.",
             productIdentifier, caseIdentifier, taskIdentifier));
 
-    taskInstanceEntity.setExecutedOn(LocalDateTime.now(Clock.systemUTC()));
-    taskInstanceEntity.setExecutedBy(UserContextHolder.checkedGetUser());
+    if (executed) {
+      taskInstanceEntity.setExecutedOn(LocalDateTime.now(Clock.systemUTC()));
+      taskInstanceEntity.setExecutedBy(UserContextHolder.checkedGetUser());
+    }
+    else {
+      taskInstanceEntity.setExecutedOn(null);
+      taskInstanceEntity.setExecutedBy(null);
+    }
 
     taskInstanceRepository.save(taskInstanceEntity);
 
diff --git a/service/src/main/java/io/mifos/portfolio/service/rest/TaskInstanceRestController.java b/service/src/main/java/io/mifos/portfolio/service/rest/TaskInstanceRestController.java
index 2180b7e..001d60a 100644
--- a/service/src/main/java/io/mifos/portfolio/service/rest/TaskInstanceRestController.java
+++ b/service/src/main/java/io/mifos/portfolio/service/rest/TaskInstanceRestController.java
@@ -17,9 +17,12 @@
 
 import io.mifos.anubis.annotation.AcceptedTokenType;
 import io.mifos.anubis.annotation.Permittable;
+import io.mifos.core.api.util.UserContextHolder;
 import io.mifos.core.command.gateway.CommandGateway;
 import io.mifos.core.lang.ServiceException;
 import io.mifos.portfolio.api.v1.PermittableGroupIds;
+import io.mifos.portfolio.api.v1.domain.Case;
+import io.mifos.portfolio.api.v1.domain.TaskDefinition;
 import io.mifos.portfolio.api.v1.domain.TaskInstance;
 import io.mifos.portfolio.service.internal.command.ChangeTaskInstanceCommand;
 import io.mifos.portfolio.service.internal.command.ExecuteTaskInstanceCommand;
@@ -71,7 +74,7 @@
                                         @PathVariable("caseidentifier") final String caseIdentifier,
                                         @RequestParam(value = "includeExecuted", required = false) final Boolean includeExecuted)
   {
-    checkCaseExists(productIdentifier, caseIdentifier);
+    checkedGetCase(productIdentifier, caseIdentifier);
 
     return taskInstanceService.findAllEntities(productIdentifier, caseIdentifier, includeExecuted);
   }
@@ -85,10 +88,10 @@
   )
   public @ResponseBody
   TaskInstance getTaskForCase(@PathVariable("productidentifier") final String productIdentifier,
-                 @PathVariable("caseidentifier") final String caseIdentifier,
-                 @PathVariable("taskidentifier") final String taskIdentifier)
+                              @PathVariable("caseidentifier") final String caseIdentifier,
+                              @PathVariable("taskidentifier") final String taskIdentifier)
   {
-    checkCaseExists(productIdentifier, caseIdentifier);
+    checkedGetCase(productIdentifier, caseIdentifier);
 
     return taskInstanceService.findByIdentifier(productIdentifier, caseIdentifier, taskIdentifier).orElseThrow(
         () -> ServiceException.notFound("No task instance ''{0}.{1}.{2}'' found.", productIdentifier, caseIdentifier, taskIdentifier));
@@ -107,9 +110,9 @@
                                          @PathVariable("taskidentifier") final String taskIdentifier,
                                          @RequestBody @Valid final TaskInstance instance)
   {
-    checkCaseExists(productIdentifier, caseIdentifier);
+    checkedGetCase(productIdentifier, caseIdentifier);
 
-    checkTaskDefinitionExists(productIdentifier, taskIdentifier);
+    checkedGetTaskDefinition(productIdentifier, taskIdentifier);
 
     if (!taskIdentifier.equals(instance.getTaskIdentifier()))
       throw ServiceException.badRequest("Instance identifiers may not be changed.");
@@ -132,21 +135,26 @@
                                          @PathVariable("taskidentifier") final String taskIdentifier,
                                          @RequestBody @Valid final Boolean executed)
   {
-    checkTaskDefinitionExists(productIdentifier, taskIdentifier);
+    final TaskDefinition taskDefinition = checkedGetTaskDefinition(productIdentifier, taskIdentifier);
+    if (taskDefinition.getFourEyes()) {
+      final Case customerCase = checkedGetCase(productIdentifier, caseIdentifier);
+      if (UserContextHolder.checkedGetUser().equals(customerCase.getCreatedBy()))
+        throw ServiceException.conflict("Signing user must be different than case creator.");
+    }
 
     commandGateway.process(new ExecuteTaskInstanceCommand(productIdentifier, caseIdentifier, taskIdentifier, executed));
 
     return ResponseEntity.accepted().build();
   }
 
-  private void checkTaskDefinitionExists(final String productIdentifier,
-                                         final String taskDefinitionIdentifier) throws ServiceException {
-    taskDefinitionService.findByIdentifier(productIdentifier, taskDefinitionIdentifier)
+  private TaskDefinition checkedGetTaskDefinition(final String productIdentifier,
+                                                  final String taskDefinitionIdentifier) throws ServiceException {
+    return taskDefinitionService.findByIdentifier(productIdentifier, taskDefinitionIdentifier)
         .orElseThrow(() -> ServiceException.notFound("No task with the identifier ''{0}.{1}'' exists.", productIdentifier, taskDefinitionIdentifier));
   }
 
-  private void checkCaseExists(final String productIdentifier, final String caseIdentifier) throws ServiceException {
-    caseService.findByIdentifier(productIdentifier, caseIdentifier)
-        .orElseThrow(() -> ServiceException.notFound("Case ''{0}.{2}'' does not exist.", productIdentifier, caseIdentifier));
+  private Case checkedGetCase(final String productIdentifier, final String caseIdentifier) throws ServiceException {
+    return caseService.findByIdentifier(productIdentifier, caseIdentifier)
+        .orElseThrow(() -> ServiceException.notFound("Case ''{0}.{1}'' does not exist.", productIdentifier, caseIdentifier));
   }
 }