Let's squash this thing about dispatching into discretion. It's perfectly legal. See FOM, Chap 21 Pg 35 para 18.1. I agree with Menard in that if they went to JFK with 4 pilots it would be reasonable to invoke Annex 5 to the FTL. So long as they complied with all the requirements and departed by 1200 local then discretion is not a player.
Have a nice day