WRT web client, make sure the browser itself supports 4K. Plex support for 4K via web browsers was only recently added (v3.69.1). Login via plex.tv, not directly to PMS IP address, to make sure you’re using latest version of Plex Web.
If on PC/Mac, try Plex Media Player instead of Plex Web. It has less limitations than a browser and will direct play more media.
Do you have subtitles enabled? If so, what happens when they are disabled? Enabling subtitles can cause video transcodes on some clients.
What clients, besides web, and make/model of attached TVs / Monitors / etc?
What is shown in Status -> Now Playing for each type of client when playing 4K media? What is transcoding, video, audio, or both?
