I have a Mac, a 4k Fire Stick and a MS DOS FAT USB stick, and playing 4k movies has proven unsuccessful so far. I followed this article step-by-step (please look at the Mac section) http://www.aftvnews.com/how-to-play-files-larger-than-4gb-off-the-fire-tvs-fat32-usb-storage-in-kodi/ and it didn't work for my MS DOS FAT USB. Whenever I go on my Fire Stick on Kodi, the folders don't detect any videos on apps like Kodi and VLC. On ES File Explorer, the video can't play although the parts are there. My USB is a SanDisk 64gb. I am using a micro USB splitter for the Fire Stick. The file works fine when I play it on my Mac from the USB.
Software/Hardware used: Kodi, fire stick, MKV, 4k, simplyrar, rar, mac