First Upgrade to RU6 and then upgrade to RU6Mp1. RU6MP1 is a mention patch and not a maintenance release, A MP can only be installed on the main build. You can install RU6 over RTM or MR3 or MR4MP2 but not RU6MP1. For that you need to have RU6
It is not recommend to upgrade directly from MR4MP2 to RU6MP1
Migrating to Symantec Endpoint Protection 11.0 RU6a
http://www.symantec.com/business/support/index?page=content&id=TECH131653&locale=en_US
Migrating to Symantec Endpoint Protection 11.0.6100 (RU6 MP1)
http://www.symantec.com/business/support/index?page=content&id=TECH137982&locale=en_US
This migration path is not supported.
Possible effects after a direct upgrade to RU6MP1 from a lower version [except RU6]:
Schema Corruption.
Possible dll corruption.
Policies Corruption.
Network breakdown.
Challenges with uninstalling SEP.
So what you need Upgrade the clients to RU6 and then upgrade to RU6MP1.