I don't think that the 970 is really powerful enough to deal with 4K. I don't think memory size is the issue. The 900 Series cards struggle at higher resolutions. Their narrow memory buses hurt them. Texture compression tricks can only get you so far. Plus 4K gaming, even with a 970, isn't spectacular. You need multiple GPUs.
Personally, I would wait. The RX 3XX series are dropping soon and rumors say that the memory bandwidth and speed are gonna be immense. Really targeting 4K. Then there is the 980 Ti. You're prob gonna need something like that to get a satisfactory gaming experience on one GPU. Wait until GTA V drops and see.
People seem to be having a pretty good time running 4K with a GTX 970 4GB with medium to high settings. The CPU won't be much of a bottleneck for either variant of the 970 either. If you're wanting it to last as long as your 460 however and don't mind spending what I'm guessing will be an extra 100-200USD for the extra VRAM, go it.