Just something to check fairly quickly - if it is not this, you need to wait for someone who knows about the plex app on PS4
On the server in Media Manager, Settings , Server, General - is it signed in to the plex.tv account ? and is this the same account that the Plex for PlayStation App is signed into
I believe the plex app asks plex.tv where the Plex Media Server is with requests like these:
plex.tv/pms/servers
plex.tv/pms/resources.xml
and the returned xml would give the plex for playstation app the local IP Address of the Plex Media Server - but for this to work, the Plex Media Server needs to be signed in to plex.tv withing the settings page