SPMC, one of the most popular Kodi forks, has added support for 4K playback and refresh rate switching on the Amazon Fire TV to a new test version. MrMC was the first to bring these much sought after features to the Fire TV, and now SPMC is following suit. There are still some bugs to workout, but if you’d like to try it out on your own device, you can find the latest test version here. You can install this test version alongside the stable SPMC version, without overriding or effecting an existing SPMC installation, because the two versions have different package names, and will therefore be treated as two different apps by the Fire TV.
When all the bugs are worked out, the final version with 4K and refresh rate switching support on the Fire TV will be version 16.5.0, so keep an eye out for that if you would rather wait for the stable public version. If you have the means, please consider donating to koying, the sole developer of SPMC, to thank him for his continued support of Fire TV devices.