4K is awful heavy on the GPU so you shop the usual Nvidia 1080. As to if the GPU helps with editing you have to check if your editor supports this or not.
4K editing will chew up any machine you can think of today. Basically you trade power and money for reductions in delays and time spent. If this is just for hobby, then you can go cheap and just wait.
However 4K UHD playback is still mindbending heavy work for the GPU today so this is why I say you have to get the most you can in that slot.
I've bought this mobo cpu combo already and i'm looking for recommendations to finish off the build. Using this for 4k video editing with premiere pro, looking to buy up to date parts that will withstand a good 2 years of heavy editing. Need suggestions for GPU and budget friendly memory (Going with ballistic sport LT 16gb of ram. How much ssd/hdd should i get to start?) not exactly sure what makes a gpu better for editing and why but I would like to hear from someone who does (i'm capped out at 400$ for the gpu)