Should I upgrade to 4k? Or does my set up need work?

[Conina]

only 3.5 GB fast video memory will be problematic in most modern PC games run in 4K. The 2 graphics cards can't share the 7 GB fast memory.

One 6GB-GTX980 Ti will probably deliver better results than two GTX970s, even if the games have good SLI-scaling.