[SPARK-51781] Update README.md and integration test with Apache Spark 4.0.0 RC4

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

This PR aims to update `README.md` and GitHub Action integration test with Apache Spark 4.0.0 RC4.

### Why are the changes needed?

To make it sure that `Spark Connect for Swift` library is ready for Apache Spark 4.0.0.

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

No.

### How was this patch tested?

Pass the CIs.

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

No.

Closes #51 from dongjoon-hyun/SPARK-51781.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

.github/workflows/build_and_test.yml

@@ -82,7 +82,7 @@ jobs:
         swift-version: "6"
     - name: Test
       run: |
-        curl -LO https://dist.apache.org/repos/dist/dev/spark/v4.0.0-rc3-bin/spark-4.0.0-bin-hadoop3.tgz
+        curl -LO https://dist.apache.org/repos/dist/dev/spark/v4.0.0-rc4-bin/spark-4.0.0-bin-hadoop3.tgz
         tar xvfz spark-4.0.0-bin-hadoop3.tgz
         cd spark-4.0.0-bin-hadoop3/sbin
         ./start-connect-server.sh

README.md

@@ -6,10 +6,10 @@
 
 This is an experimental Swift library to show how to connect to a remote Apache Spark Connect Server and run SQL statements to manipulate remote data.
 
-So far, this library project is tracking the upstream changes like the [Apache Spark](https://spark.apache.org) 4.0.0 RC3 release and [Apache Arrow](https://arrow.apache.org) project's Swift-support.
+So far, this library project is tracking the upstream changes like the [Apache Spark](https://spark.apache.org) 4.0.0 RC4 release and [Apache Arrow](https://arrow.apache.org) project's Swift-support.
 
 ## Requirement
-- [Apache Spark 4.0.0 RC3 (March 2025)](https://dist.apache.org/repos/dist/dev/spark/v4.0.0-rc3-bin/)
+- [Apache Spark 4.0.0 RC4 (April 2025)](https://dist.apache.org/repos/dist/dev/spark/v4.0.0-rc4-bin/)
 - [Swift 6.0 (2024)](https://swift.org)
 - [gRPC Swift 2.1 (March 2025)](https://github.com/grpc/grpc-swift/releases/tag/2.1.2)
 - [gRPC Swift Protobuf 1.1 (March 2025)](https://github.com/grpc/grpc-swift-protobuf/releases/tag/1.1.0)