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.
thanks again,
John
EveryMac has excellent details on all macs.
If you drill down by Capability, you will find the resolution supported by your graphics card