Mac Pro Late 2013 and 4k issues

Hi,

So I have the older Mac Pro 12 core with 32gigs of ram and the two Firepro d700 graphics cards. I just bought a 4k monitor and can't for life of me get it to work. I have it plugged into the HDMI port on both the Mac and the monitor with nothing plugged into the other Thuderbolt ports. Yet when the computer comes up the only resolutions available are 5120x1440 (low resolution) or 1920x1080 and 1080p and 1080i. If I select either the native resolution or 5120x1440 option the image is horrible. The only resolution setting that works is the 1920x1080, however, since it is a large monitor everything is huge. After looking on the internet it seems that my Mac Pro should be able to output 4k so I'm not sure why it isn't working. I've tried 4 different HDMI high speed 18gbs cables as well as plugging it into one of the Thuderbolt slots. I even downgraded the OS to Yosemite which then at least fills the screen but it's still at a odd resolution. Any thoughts would be helpful.

Thank you