To resolve all the issues, we can recommend you Pavtube iMedia Converter for Mac, which is an all-in-one tool that can help you to convert and downscale 4K videos within one stop. Overall, this program is a completely professional 4K video converter and player. Just like the way it is dealing with 1080p videos, it is surely in the support of converting 4K videos in any formats faultlessly. With it, you can fast convert 4K Ultra HD videos in MP4, MKV, MXF, XAVC, WMV, AVI, or any other extension to QuickTime playable format with least quality loss. Plus, this 4K video converting software provides the optimal presets for portable devices like iPad, iPhone, Galaxy Tablet, Kindle Fire HDX; editing programs like Adobe Premiere Pro, Final Cut Pro, iMovie, Avid MC, etc for better enjoying, editing 4K files. It is also available to customize the output resolutions to any size the users want.

May you need player app.

I dont think MX6 support to play 4K videos

U can try some decoder, like a "HandBrake" =/