Hey,
i just get Unreal Engine4 for my MacBook 2013 with Yosemite,
sometimes there are small Screen Problems, there is a flickering at my Screen (like a old tv)
Also the Engine froze every time for 5 Minutes if a add a Material 
Is there any Solution for that?
Mfg, Simon
If you see momentary black flickering on the screen when using an Intel GPU this is already known & logged internally as [UE-6124] & it appears to be a driver bug so has been filed with Apple. There has been no suggested solution.
There are other open AnswerHub topics about material/texture freezes - in summary they are perfectly normal behaviour in UE4.
In case you have mac with a dedicated and an internal GPU:
Even if this a driver Problem, the loading time can be reduced if you switch off the automatic change betwen the dedicated and integrated GPU in your Energy Settings (System > Preferences > Energy Settings)