I am not understanding your issue. You have a NAS (but don’t state what OS your are running). I am guessing it is running your Plex and Jellyfin dockers to catalogue and and it is serving up media to your “fancy new TV”. On that TV you should have Plex or another client like Kodi installed on your TV to play your media. If it is only 4K that is not playing well, then it could be your network speed, or your server is trying to transcode when it should be sending native 4K. Hope that helps but more details of your server, network, and clients would help.