Great, thank you so much Chris, I wasn't aware of this site.
If my graphic cards do not support 4K do you think it is possible to put one in a say 2009 Mac Pro that does support it? I'm ignorant in replacing graphics cards because I've never had to do it. If I'm going to buy a 4K display I'd better find out.
EveryMac has excellent details on all macs.
If you drill down by Capability, you will find the resolution supported by your graphics card