I wouldn't bother trying to chase 4k. you'll need two gpu's, and you won't be happy when games don't have SLI support
try to get a 1440p gsync monitor and a gtx 980 or better. if you get something like a gtx 970, I would stick to 1080p
to be clear, you can do 4k 60fps in a lot of older, or less demanding, games. but newer graphically demanding games will be a problem. better to have a 1080p or 1440p monitor, and just downsample if you have a lot of overhead. also a 144hz gsync monitor will be able to take advantage of higher (60+) framerates