For SCOM 2012 SP1 there are 14 fixes. For UNIX/Linux monitoring there are 6 additional fixes as well.
My advice 01: be careful with rolling out this Update Rollup since previous versions introduced sometimes new challenges as well. So it’s better to test this update in a test environment and wait with moving it to production until this update is longer available and update experiences are posted on the internet.
The Update Rollup 4 package for SCOM 2012 SP1 can be downloaded from here.