[brazen1]

Your "cheapest option for 4k HEVC playback" (you mentioned .mkv files) is a PC provided you have a somewhat modern GPU (video card) in it.  Mine is a few years old.  Then you can play anything and everything you want audio and video at the highest quality.  You don't need a new specific device considering you might already have one (a PC) on hand unless you also require convenience but even that requires adjusting.  If you don't have a PC with a half way decent GPU, you might be able to obtain an old one from a friend or neighbor possibly.  People are always looking to get rid of them which save them the hassle of recycling.  The GPU you may have to purchase new or used and might be less money than a device that will have caveats and limitations no matter which one you consider.  A GTX 960 is or better is sufficient as are AMD models.  I would avoid Intel Graphics personally.  This approach would require some reading and following directions for a perfect and reliable outcome.  The software involved is free unless later you decide to unbridle more functions.  Lots of folks simply use the streaming app in their display vs a dedicated device designed with streamers in mind.  While PC's can stream, they are not the best option.  The apps in your display or a device cater to streaming better.  There are many guides to help folks.  Mine is my signature should you be interested.