My son runs Plex from a Pi 4 now. Works fine, even transcoding on the fly so he can watch stuff on his phone over t'internet. You probably know that the Pi 4 runs very hot, so it's definitely worth getting a decent active cooler . He's got one of
these and the Pi seldom goes over 50C even flat out.
This bloke is doing the Pi 4 for £10 less than most places right now and if you're interested in auctions, there's a couple on ebay right now.
EDITED: 14 Apr 2020 12:50 by WILLIAMA