If you would like MR-1938 patch (see link below), "Ability for having user's classes take precedence over the system classes for tasks' classpath", to be included in CDH3b4 release, please put in a vote on https://issues.cloudera.org/browse/DISTRO-64.
The details about the fix are here: https://issues.apache.org/jira/browse/MAPREDUCE-1938 Roger
