Could you share some details about how you do this? Do you need displayport on both ends, and is it a monitor or TV? I have the same machine and a 4K TV with only HDMI-inputs and am curious if I can achieve this is in some way. Switchresx looks really useful anyway, so thank you for that. :)

If you have a Displayport display, you can use a displayport to mini displayport cable, use switchresx, should work good.

If it's HDMI only, as the sibling pointed out the HDMI port won't work. However, Startech makes a displayport to HDMI adapter which works ( https://www.amazon.com/DisplayPort-HDMI-Adapter-Converter-60... ) (just make sure you get one which supports 4k60hz, most DP -> HDMI adapters don't).

I'll consider this though I it would be probably be sitting unused after trying it anyway as I really have no need for it. Thank you for the information. Just to clarify, we're both talking about a late 2013 macbook pro retina? The other helpful commenter mentions the 2015.

Yeah, this is all on the late 2013 15" with the nvidia dgpu - https://support.apple.com/en-us/HT206587 has the official details on this BTW - they state that SST Displayport 3840 x 2160 displays are only supported at 60hz on the 15" from Mid 2014 on, but it actually works on the Late 2013 also with Switchresx.

There's no HDMI 2.0 support in 2015 MBPr AFAIK. So no 4k@60Hz over HDMI. You must get DP.