Watching personal 4K videos on Apple TV 4K
Apple limits the resolution of airplay.
You need to install a player and transfer the video to the player documents folder.
Use itunes or imazing, wired  or wireless transmission.Then you can open the video in the player.

Another method is to install a player that supports network sharing, turn on file sharing(SMB NFS DLNA) on the device where the video is located(PC MAC NAS). Mount the network sharing folder, you can stream it at 4K.