Are the two cores of rMPB enough?
It depends on how you're going to use it.

If your daily MacBook usage only consists of stuff like websites browsing, watching movies on VLC and creating a Word or Excel document from time to time, then, I would say, 2 cores are surely enough. 

But if you're using software that requires more resources like audio/video processing (Adobe products, for example), graphic excessive games, etc. then go for 4 cores.