Reduce the CI parallelism to speedup flow

The current number of available slaves on our CI is
too low to allow a high parallelism of changes validations.

Temporarily lower the threshold to speedup the executions
of the changes validations in queue.

Change-Id: I98450d42dfcbcf99225fc92b19c493457bf530e8
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 8455cd9..4779d63 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -33,7 +33,7 @@
   static int numRetryBuilds = 3
   static int myAccountId = 1022687
   static int waitForResultTimeout = 10000
-  static int maxBuilds = 15
+  static int maxBuilds = 4
   static String verifierJobName = "Gerrit-verifier-change"
 }