Hello Jay,
The 2012 Mac Pro is very versatile when it comes to monitors. What graphics card do you have in your Mac Pro? If it's the original ATI Radeon HD 5770 or 5870, then you'll need to upgrade your Graphics card to run 4k, as ATI says the maximum resolution of those cards is 2560 x 1600.
My recommendation would be to find a 4k monitor that suits your needs and budget, and depending on what you do with your computer, buy a GPU that suits your needs.
If you don't use your GPU that much, an AMD RX 560 would be more than adequate, but if you rely on it more, an AMD RX 580 may be a better fit. Either way, both cards are much faster than the original card.
Hope it helps!
-Geotrax