[wrxtasy]

To understand because i am not very familiar with ATV4 & MrMC..
It can play all x265 hevc 10bit hdr & x264 files?

Yes for the ATV 4K, just throw content at it, handles anything I've tried 1080p Hi10P Anime included, even High Bitrate 4K video over 5GHz N Band WiFi.
You can use other media player Apps like Infuse that are even easier to setup, but less flexible compared to a Kodi fork like MrMC

Is there an additional charge to install MrMC on Apple TV 4k?

The unrestricted version is US$6.99

I have been sitting on the fence for years looking at all these various Apple TV's that have come out, but none of them had the Operating System & Firmware smarts to handle video properly.
This 4K model has been on the market for not even 3 months and already it is superior to any other App type media player on the market IMHO. It just works. Set and forget.
Apple have actually listened to video enthusiasts for once and gone on the bug busting hunt.

Hell it even has legacy audio support for decoding Dolby Digital Plus audio you get from Apps like Netflix and Amazon Video and extracting that to DD/AC3 to support older HDMI audio receivers. Try doing that on officially licensed Android TV media players and you come up empty handed.
All that may be needed is Dolby Atmos and DTS:X passthrough audio support, but not many people have that sort of expensive AVR & speaker setup anyway.