I edit video (inc'l 4k) professionally, mainly using FCP X but also Premiere Pro CC. My main machines are 2013 and 2015 top-spec iMac 27s. 4k video is incredibly demanding. Ideally you want a top-spec iMac, 2013 or later. Of course a new Mac Pro can be better but to generally exceed the 2015 iMac 27 performance requires an 8-core or above D700 model.
A major issue is what editing software you use, and whether you will ever edit multicam. On identical iMac hardware, FCP X is much faster than Premiere Pro CC on many common editing operations when processing H264 4k material. E.g, using JKL commands FCP X can go from fast forward to reverse in a fraction of a second; it takes Premiere several seconds to respond. The FCP X viewer update rate is about 20x faster when fast forwarding in the timeline.
FCP X has seamless built-in proxy workflow which becomes very important for 4k. On 1080p you generally don't need proxy files for either FCP X or Premiere if on a powerful machine. However 4k is different, and especially 4k multicam.
Even on a 2015 top-spec iMac 27, you generally need proxy files for H264 4k multicam. Premiere doesn't have this so you'd have to manually transcode externally and move files around. All this feeds into your hardware decision.
If you are using FCP X, a top-spec 2013 iMac 27 or later is OK for H264 4k, including multicam. If you are using Premiere I would probably not suggest using an iMac but get a powerful Windows machine with a GTX-980 Ti or above. That can allow smooth, responsive editing of camera native H264 4k files without transcoding.