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.