Divx free converter encodes 4k-h.265-MKV videos. It also has a player so you can view the videos on you Mac:
Click here to go to Divx web site...
(Been using it for years.)
HandBreak can also make 4k-h.265-MKV and MP4 videos. HandBreak is free:
Click here to go to HandBreak web site...
You can use the DivX player to play the HandBreak 4k-h.265 videos.You can download the DivX Player as a stand-alone app from the above link. If you wish to download the entire DivX package click on the yellow DOWNLOAD button on the download page.
The DivX encoder has more setting options compared to HandBreak. I've gotten some good results with the DivX encoder... 4k h.265 video compressed down to 12,000 kbits/sec. Try the HandBreak encoder to get yourself going, easy setup and down the line try the DivX encoder.