diff --git a/mysql-test/mysql-test-run.pl b/mysql-test/mysql-test-run.pl
index ae277c4a554..0fdfcb467f2 100755
--- a/mysql-test/mysql-test-run.pl
+++ b/mysql-test/mysql-test-run.pl
@@ -496,14 +496,25 @@ sub command_line_setup () {
   my $im_mysqld1_port=   9312;
   my $im_mysqld2_port=   9314;
 
+  #
+  # To make it easier for different devs to work on the same host,
+  # an environment variable can be used to control all ports. A small
+  # number is to be used, 0 - 16 or similar.
+  #
+  # Note the MASTER_MYPORT has to be set the same in all 4.x and 5.x
+  # versions of this script, else a 4.0 test run might conflict with a
+  # 5.1 test run, even if different MTR_BUILD_THREAD is used. This means
+  # all port numbers might not be used in this version of the script.
+  #
   if ( $ENV{'MTR_BUILD_THREAD'} )
   {
-    $opt_master_myport=   $ENV{'MTR_BUILD_THREAD'} * 40 + 8120;
-    $opt_slave_myport=    $opt_master_myport + 16;
-    $opt_ndbcluster_port= $opt_master_myport + 24;
-    $im_port=             $opt_master_myport + 10;
-    $im_mysqld1_port=     $opt_master_myport + 12;
-    $im_mysqld2_port=     $opt_master_myport + 14;
+    # Up to two masters, up to three slaves
+    $opt_master_myport=   $ENV{'MTR_BUILD_THREAD'} * 10 + 10000; # and 1
+    $opt_slave_myport=    $opt_master_myport + 2;
+    $opt_ndbcluster_port= $opt_master_myport + 3;  # and 4 5
+    $im_port=             $opt_master_myport + 6;
+    $im_mysqld1_port=     $opt_master_myport + 7;
+    $im_mysqld2_port=     $opt_master_myport + 8;
   }
 
   # Read the command line
diff --git a/mysql-test/mysql-test-run.sh b/mysql-test/mysql-test-run.sh
index ac5457beaa6..83b91dd721d 100644
--- a/mysql-test/mysql-test-run.sh
+++ b/mysql-test/mysql-test-run.sh
@@ -244,8 +244,13 @@ MYSQL_MANAGER_USER=root
 # an environment variable can be used to control all ports. A small
 # number is to be used, 0 - 16 or similar.
 #
+# Note the MASTER_MYPORT has to be set the same in all 4.x and 5.x
+# versions of this script, else a 4.0 test run might conflict with a
+# 5.1 test run, even if different MTR_BUILD_THREAD is used. This means
+# all port numbers might not be used in this version of the script.
+#
 if [ -n "$MTR_BUILD_THREAD" ] ; then
-  MASTER_MYPORT=`expr $MTR_BUILD_THREAD '*' 5 + 10000`
+  MASTER_MYPORT=`expr $MTR_BUILD_THREAD '*' 10 + 10000`
   MYSQL_MANAGER_PORT=`expr $MASTER_MYPORT + 2`
   SLAVE_MYPORT=`expr $MASTER_MYPORT + 3`
   NDBCLUSTER_PORT=`expr $MASTER_MYPORT + 4`