Considering Plex website stills only lists version 1 of PMP for embedded devices, unless v2 is available since ages and works really well on Pi3, I don’t expect much Plex guys to move their ass to get a version compatible with Pi4 (and it also depends of avalaibility of Librelec on that new platform too but there is already an alpha for it according to Librelec blog !
We’ll be probably stuck with Pi3 and current v2 of PMP for months I guess
Unless it’s a really nice and cheap platform to run PMP, I guess most users don’t go this way with geek boards so Plex don’t prioritise it in dev !