The late 2014 iMac 5K has a 5120 X 2880 retina display.
But don’t expect to see an external 5K display any time soon: even Thunderbolt 2 cannot deliver the bandwidth needed for 5K, and that’s even 8 bit color (vs 10 bit color, for which OS X still has no graphics drivers).
Bandwidth: 5120 X 2880 X 3 bytes = ~44MB/frame @ 60 Hz = 2.654 GB/sec, versus 3840 X 2160 X 3 bytes = ~25MB/frame = 1.49 GB/sec for a 4K UltraHD display. Thunderbolt 2 can handle a 4K display but that’s where it maxes out.
As far as MPG understands, Thundebolt 2 is limited to 20 Gbps bandwidth, or 2.5GB/sec, so 5K video is a bit too much.