I use an older QNAP (TS-420) and like it a lot. AVS Forums seems to like Synology more for what that's worth.
I'd recommend that you put your Plex server on a Windows PC machine for two reasons, (1) the windows version is updated more often, (2) the NAS will not be as powerful as your basic windows machine.
If you're just doing music, then you're okay running on the NAS; but if you are considering video streaming then use the PC for Plex.
So yeah, I think you're on the right path. You may not need a 4-bay just for music, but having the 4 bays would let you run RAID5 which allows for a single disk loss which'll cover you in most cases. And Plex is awesome. You can always play music straight from the NAS, but Plex will give you playlists and searching and all that stuff. (QNAP has a music streaming add-on; but it's not as freindly as Plex.)
