What Mac for 4K external display

I am looking for a source for a 4K external display from any Mac device, I was hoping to use an iMac so that the both the iMac display and the external display can run at 4096 x 2160 simultaneously. I have not been able to get a satisfactory answer from anyone at Apple yet.

The external display is via an Analog way Ascender 32 to a 4K projector.