I’ve recently jumped ship from an iMac 27" (late 2013, Core i5) to a custom built gaming PC as the iMac simply cannot be used with UE4. It was fine with Unity, however there are other tools that stretch it too, like Substance Painter.
I hate Windows; it’s cr@p compared to OSX, however my only other choice is to build a Hackintosh PC and I cannot be bothered with the additional effort involved in maintaining it (I’ve done it in the past). So back to Windows I go 
Incidentally I paid £870, including VAT for i5 6600K, 16GB 2133MHz RAM, GTX 960 4GB and a 480GB SSD. Looking at the previous posts in this thread, this should run UE4 very well.