No, you don't HAVE to reserve 4 months out. NCL opens only about 30% of the available space for pre-cruise booking so if you don't do it ASAP you MAY be frustrated that you can't book the day/time you want. The rest is available on embarkation day - visit, I believe, Teppanyaki on the Breakaway ships, as soon as you board to make reservations, or you can try using the app on your phone once you're connected to the ship's WiFi (no need to use internet package for that), or you can use the touchscreens in the stair lobbies (2 on each deck at each end of the ship on the BA and BA+ ships) to make those reservations.