^^ Word.
980ti or 980 SLI on any of my systems with my 4k Acer 28" Gsync, if I run the game at 4k (Beta especially I'm talking here), FPS can drop into the 40s easily- even at 1440p or 1080p with everything maxed in heavy areas, it can drop into the 50s at times when low over tank town/example with cons around. Also be aware that at 4k the planes/targets get much smaller - while looking great, you might find hitting things becomes harder until you get accustomed to the higher res.
Sure looks great though, the models/terrain in beta is something I'm just starting to appreciate - going to be some really "wow" screen shots and films coming out.
Also - I would recommend waiting a couple more months, nVidia's new 1080 cards will be out by then, plus another couple months after that the new HBM memory GPUs from both AMD and nVidia will be out, and they'll likely smoke the 980ti - also the price of the 980ti will drop very soon due to the new cards coming out. If patience isn't a virtue like me, nothing wrong with getting a 980ti now, just be aware that it's near the end of the cycle, and it'll not be 'top dog" for more than another couple months (May/June is the latest from nVidia for 1080 release). Also, Kaby lake (Skylake + pretty much) and Broadwell E CPUs will be out roughly this late spring/summer too - so if you want the latest, waiting for another couple/few months wouldn't be a bad decision. There will also be a lot more info from players here about how 4k/etc is working with the new version by then.