Updated image MXIII_Plus (20170608). Will probably be 4k. I have no such equipment, so I have not tested this possibility.
No big deal, it's just a nice to have feature for future. Actually, I have no 4k content at the moment.
Testresult... 4k still not working.
Btw, a couple of month ago, I installed Wetek Core on the MX3plus (same CPU and GPU). Beside the sideload menu and IR remote was broken (bluetooth and wifi not tested, ethernet was ok), it run at 4k. Maybe this info helps to fix the problem.