ci: bump container count for unittests (#31490)

[skip ci]
diff --git a/.github/workflows/server-tests-mariadb.yml b/.github/workflows/server-tests-mariadb.yml
index cdb6849..91a0114 100644
--- a/.github/workflows/server-tests-mariadb.yml
+++ b/.github/workflows/server-tests-mariadb.yml
@@ -39,7 +39,7 @@
       fail-fast: false
 
       matrix:
-        container: [1, 2, 3]
+        container: [1, 2, 3, 4]
 
     name: Python Unit Tests