[MPLUGIN-441] Upgrade to maven alpha-4 (#191)

diff --git a/maven-plugin-plugin/src/it/v4api/verify.groovy b/maven-plugin-plugin/src/it/v4api/verify.groovy
index c3c4967..59a369a 100644
--- a/maven-plugin-plugin/src/it/v4api/verify.groovy
+++ b/maven-plugin-plugin/src/it/v4api/verify.groovy
@@ -23,7 +23,7 @@
 def pluginDescriptor = new XmlParser().parse( descriptorFile );
 
 assert pluginDescriptor.requiredJavaVersion.text() == '1.8'
-assert pluginDescriptor.requiredMavenVersion.text() == '4.0.0-alpha-2'
+assert pluginDescriptor.requiredMavenVersion.text() == '4.0.0-alpha-4'
 
 def mojo = pluginDescriptor.mojos.mojo.findAll{ it.goal.text() == "first" }[0]
 
diff --git a/pom.xml b/pom.xml
index ff15453..2519ecd 100644
--- a/pom.xml
+++ b/pom.xml
@@ -92,7 +92,7 @@
   <properties>
     <javaVersion>8</javaVersion>
     <pluginTestingHarnessVersion>3.3.0</pluginTestingHarnessVersion>
-    <maven4Version>4.0.0-alpha-2</maven4Version>
+    <maven4Version>4.0.0-alpha-4</maven4Version>
     <mavenVersion>3.2.5</mavenVersion>
     <!-- SLF4J version must match the version exported from the mavenVersion, 
     https://github.com/apache/maven/blob/12a6b3acb947671f09b81f49094c53f426d8cea1/pom.xml#L63 -->