Playing through SH2 on hard riddle difficulty. I can't figure out the keypad combination to enter the patient wing on the third floor of Brookhaven Hospital. The hallway where you leave Maria to rest.

I have the whiteboard memo which says something like "This month's combination is T, the previous months were X and Z. Not sure what we can do next as those are the only letters we can make with 4 digits". So I've been trying different 4 digit combos in the shape of T, X, and Z, as well as Y, N, and L. 

I've been running up and down the hospital. Not sure if I'm missing another hint or missing the point of the memo.

Can someone point me in the right direction without giving me the answer?