buying a new GPU

I'd go for the 980 Ti if you plan to go 4K.

The 980Ti is by far the better card but 4K is very hard on GPU's so even it is not enough for true 4K gaming without lowering the image settings. Many people who want to run 4K have to get 2x 980Ti's in SLI. Pascal is coming soon and is supposed to be more 4K oriented and AMD is coming out with Polaris which is also supposedly 4K oriented. This is just my opinion and it could be way off. Anyone else have any thoughts?

For 4k the 980Ti is the minimum but really you want 2x sli 980Ti's if you want to game at high settings near 60fps. What is the rest of your system including make and model of PSU?

AMD Fiji based cards handles 4k better than current nVidia cards, no question.
R9 390X or Fury will get the job done.

I'd be inclined to wait 3-4 months and get the next gen (Polaris from AMD or Pascal from nVidia)

From the benches I have seen the 980Ti/TitanX come out on top at 4k?

Yep as sizzling said, while the CX is unlikely to become a hazard (at least not an explosion-grade hazard) to other components, it'd be best if you replaced it before putting in a high performance card. And as otherwise mentioned, unless you need CUDA for something else, there is absolutely no valid reason to not get something like a Fury X instead since it's a better card in more ways than one.

Raw performance was practically the same between 980 Ti and Fury X on launch (Fury X coming out on top in some instances, 980 Ti in others) but since then new drivers and DX12's Async Compute feature (which is not universally adopted but may become more widespread when Pascal and Polaris come out) has given pretty much all non-ancient AMD cards a performance boost (the stronger the card the bigger the boost).

And the "nvidia has better drivers" and "developers favor nvidia" arguments are both practically myths. There's pretty much just as many games where AMD comes out on top, and while nVidia has historically been quicker to fix driver issues, they are not immune to screw-ups (see recent driver update screwing up nVidia systems and the "fix" not fixing much at all).

tl;dr Unless you need CUDA for programming/rendering/whatever there's no logical reason to get a 980 Ti over a Fury X.

Get a Fury X and a new decent PSU (EVGA 750B2 might be a good choice).

LIST your entire computer hardware configuration....
because a new GPU might just be a waste of money.
You NEED a CPU and motherboard which is capable of supporting the graphics.
If you don't have that, you are throwing money away for nothing.

A new GPU does NOT cure an obsolete motherboard or an obsolete processor.
Listen to what I say.