Increase the build parallelism to 4

So far the CI has been quite happy to manage up to 3 parallel
changes validation. Let's try to see if we can pump up to 4
which means > 16 concurrent builds.

Change-Id: Iacabab406b52578dcdcbfdc4f1bb77dc41cd82b9
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index ebd1c9a..35af137 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -32,7 +32,7 @@
   static int numRetryBuilds = 3
   static int myAccountId = 1022687
   static int waitForResultTimeout = 10000
-  static int maxBuilds = 3
+  static int maxBuilds = 4
   static String verifierJobName = "Gerrit-verifier-change"
 }