I’m on an iMac right now (OS X and Win 10, running Win 10 as I type this), there’s nothing to stop you dual booting using Boot Camp if you wanted to go that route, and I have UE4 installed on both, and it works on both. Performance on OSX is a little lower as of 4.10.1, I haven’t tried 4.11 Preview at all and that’s not based on hard data, just my perception from using both. UE4 also runs at Shader Model 4 level on OSX due to Apple not updating OpenGL at all over the last 3 major versions, off the top of my head the OS ships Open GL 4.1 (“with extensions”).
If you wanted a system for development in UE4 and UE4 alone I’d suggest a Windows based system right now, however if you wanted a Mac for any other reason you have to weigh up pros and cons for your particular use case.
I’ve been trying to migrate away from Windows for 2 years now and have mostly been successful, games are the only thing keeping me here and that’s more for playing them than making them.
(Edit: I also have a custom built windows PC running Win 7 and an Ubuntu box in the “naughty corner” because I made a typo, but those systems aren’t really relevant to the question)