At 4K you will still be mostly GPU limited and likely won't exceed 60FPS, so you should be fine with your current CPU. I could potentially see you bottlenecking in some of the newer Ubisoft open worlds eg. Watch Dogs 2 or AC: Origins where you need more than 4 threads to avoid dropping down to 30-40FPS in certain areas of the game. Outside of those titles, you probably will still be GPU limited.