[SPARK-56435] Upgrade `Spark` to 4.2.0-preview4

### What changes were proposed in this pull request?

This PR aims to upgrade Apache Spark to 4.2.0-preview4 as a part of preparation for the official Spark 4.2.0.

### Why are the changes needed?

Apache Spark 4.2.0-preview4 is released.
- https://github.com/apache/spark/releases/tag/v4.2.0-preview4
- https://repo1.maven.org/maven2/org/apache/spark/spark-core_2.13/4.2.0-preview4/

### Does this PR introduce _any_ user-facing change?

No behavior change.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.6

Closes #630 from dongjoon-hyun/SPARK-56435.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

gradle/libs.versions.toml

diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index d7e146f..c4acc16 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -20,7 +20,7 @@
 netty = "4.2.12.Final"
 operator-sdk = "5.3.3"
 dropwizard-metrics = "4.2.38"
-spark = "4.1.1"
+spark = "4.2.0-preview4"
 hadoop = "3.5.0"
 log4j = "2.25.4"
 slf4j = "2.0.17"