Basically you have a door you need to get through with a number keypad to unlock it. In a room is the clue on a black board that says "This months code is T, last month it was X and the month before that it was Z. But what are they going to do next month because that's all you can define with 4 numbers.
From what I can see the pattern here is offering two options... next months letter will be either L or N... I think it's L...
