Hi,
Thank you for posting in Symantec community.
Can SEPM 12.1.2015.2015 manage client of 12.1 RU4 or RU5?
--> Yes, it can. But it's not recommended.
It's always recommended to have SEPM and SEP clients on the same version.Latest release provides many other enhancements and improvements.
For RU5 clients you need to set distribution job for these clients.