I solved this problem by having all of my media in a supported format of all of my devices so no transcoding is needed. My Pi 4 can transcode two 480p streams simultaneously or a single 1080p stream but can’t do any more than that. However it can stream at least 4 streams simultaneously a mix of 4k, 1080p and 480p. It suits my needs unless I’m transcoding 4k due to subtitles 😵. Hopefully transcoding won’t be required for subtitles forever… right? Right?
My understanding is some parts have to be done sequentially even though the parts themselves are multithreaded, now the different parts can all be done in parallel.