Hi,
Thank you for posting in Symantec community.
If SEPM is on SEP 12.1 RU4 (12.1.4013.4013) then I would suggest not to upgrade only clients to SEP 12.1 RU4 MP1.
It's recommended to have SEPM and SEP clients on the same verison.
Are you trying to cover any specific bug by upgrading clients to the latest version?