Which Thunderbolt? TB 1 & 2 run on what looks like Display Port cables. TB1 can not support a 4K monitor; but TB 2 can. But when Apple releases the new Skylake based Macs this year we will see TB 3 and USB 3.1 all running on USB-C ports and cables.
So you might want to consider a plan for replacing the 2008 Mac Pro with a newer machine that does TB 1 to 3, USB 1 to 3.1, all over USB-C ports and cables. The new machines should support external 5K monitors.