Everything I'd read suggests that Avid Media Composer gets along just fine with 4K video on a (non-Pro) iMac, if you bump up to a Core i9, Radeon 580, and 32G of RAM or so. The eGPU or the iMac Pro would be nicer, but the base iMac is still powerful enough it's not going to make you hate life. There were 4K movies being edited on less powerful machines just a few years ago, after all. :)