When it comes to 4K displays, it depends on the display and your Mac Pro. Here is the support article from Apple: https://support.apple.com/en-us/HT206587
I have a Md-2014 MBP 15" and to get 4K at 60hz,I needed to go to the Mini-Display Port(works with Thunderbolt Port) to Display Port 1.2 standard. The Display Port 1.2 standard is important. I tried this with a non-compliant Mini DP to DP and had issues.