Higher resolution always look better, providing you have the graphics power to run it, the problems come when you have to turn down other video settings such as shadows or texture detail in order to maintain your higher resolution with a good frame rate.
4k is very demanding, it's 4x more pixels than 1080p and so some games don't run very well, other games fare better.
Be wary of people slamming 4k for performance however, it matters what kinds of games you play, while it's a perfectly valid observation that games such as GTAV won't run 4k easily on a single GPU, it is the exception rather than the rule, many other games run just fine in 4k with max settings and look great.
The best idea is to check out benchmarks for gmaes you play like WoW, those games graphically are pretty terrible and so should run in 4k on that GPU quite well.
You can always shoot for a resolution higher than 1080p but smaller than 4k, the next in between is 2560x1440 or 2560x1600 (depending if you want 16:9 or 16:10), this is a much easier resolution to run about 1/2 as demanding as 4k but about twice as demanding as 1080p which is a great sweet spot for something like a 970.
