I think you slightly misunderstood.
I'm not suggesting buying
two desktop machines.
Either -
1. Macbook Pro and (non Apple) 4k / 5k monitor
or
2. iMac 5K and Macbook Pro
However I've just seen this (attached) which seems to say that the Macbook Pro retina (with the Radeon R9 M370X) will support a single monitor of up to 5120 x 2880 at 60Hz (which I think is 5K)
Personally I would prefer the flexibility of a laptop and monitor rather than a desktop (so long as I can get a cordless mouse for the Macbook Pro when I'm using it with the monitor)
Does anyone know which 4K / 5K monitors are compatible ??
EDIT....This further complicates matters -
Dell UP2715K 27-Inch 5K monitor....
This monitor requires two DisplayPorts (for 5K support), a mini-DisplayPort (for 4K support This will not run at the native resolution) / so basically it requires dual DisplayPort inputs to drive its 5120x2880 panel at full resolution. Thunderbolt 3 and Display Port 1.3 will support 5K on a single cable.
Mac users be aware "Dell's upcoming 5K display uses dual DisplayPort 1.2 cables for connectivity but that no current Macs appear to support the setup and even if they did performance would likely not be ideal" The current DisplayPort 1.2 specification used over Thunderbolt 2 on Apple's latest Macs simply isn't capable of handling the bandwidth necessary for 5K video over a single cable. As a result, no current Mac, including the Mac Pro and Retina MacBook Pro models that do support 4K displays, can currently drive a 5K external display «
16 Jan 2015 by PCBuyIT Limited
Apple's information (below) therefore seems misleading.....