I would advise not to upgrade yet. At this point, most 4K content can not be transcoded properly no matter how powerful hardware you get. Wait untill tonemapping has been implemented or untill plex decides to retain 10 bit colors in the transcoded stream and decide what hardware to get then.
By upgrading now, youll move from one problem to another.
The Apple TV 4K should not need transcoding with 4K, so make sure you have it set to max or original quality. If you still have issues, its most likely a problem with your network