ScottMescudder :
Definitely the RX 480. Now if you want to do mostly 4k gaming go with the 8gb version rather than the 4gb. Although if you plan to stay 1440p and 1080p and below then go with the 4gb. You won't see any if at all a difference between the 4gb and 8gb during 1080p and 1440p. If you plan on keeping the budget for the GPU lower due to the Ryzen being close to 700$ish then go with the 4gb 480. It does just fine with 4k as I use one myself.
The RX 480 is NOT a 4k card. Maybe with the details dropped a lot and if you're willing to run below 60 fps. But it is very much not a 4k card, and hardly even a 1440p card. Also there are many games such as DOOM, Battlefield 1 and GTA V that will use all 8gb even at 1080p.
For the record I have an RX 480 as well.