Mikey,
Not sure why you brought up the nVidia Quadro P600 because DGroves and OP do not mention it at all. And, it in fact is supported on the HP Z4 G4 as detailed on page 4 of:
http://h10032.www1.hp.com/ctg/Manual/c05857891.pdf
And, on multiple pages of the HP QuickSpecs for the Z4 G4.... I'd try some other video card that is not way old. The cards HP lists in the QuickSpecs are the current generation ones that they have done their extensive conformance testing on. Many other cards not in the QuickSpecs will work just fine.