Upon further testing, seems the PS4 has a limit of 2500 entitlement (I hope this is per game, but never know. Cant really check as Rock Band 4 is the only use of my 5.05 ps4 atm). If you convert your song 1 per package, that's a maximum of 2500 added songs.
Rock Band 4 also seems to dislike big package, at first I tough it was based on amount of song in pack, but currently testing to see if it's based on size.
Edit: After more testing, it appears the pak limit is on size, keeping it under 100mb seems to provide working paks. (So about 100mb worth of con files)