PS4 Disc Requirement

[HEC]

I didn't use the free upgrade because I wanted to retain the right to play on PS3, so I bought the retail PS4 version and I noticed this.

A bit late for this now but you could have saved yourself both hassle and money by upgrading PS3 -> PS4 and then purchasing another (much cheaper) PS3 copy of the game. Unfortunately - there is not much you can do about that now. You can try to uninstall game and downloading it from PSN store instead but I'm not sure if that would help or not.