I'M LOCKED OUT ON THE ROOF!

Luckily, security isn't very tight in this building.

You can easily deduce how often each key on the keypad is used by how smudged it is.

And you can easily deduce how long the code is by just pressing randomly and waiting for the feedback sound.

A is so much more smudged than B; it's certainly been pressed much more. C and D don't appear like they've been pressed at all.

If the code is 4 digits long, then there must be 3 As and 1 B in it.

So now it's just a matter of trying out all combinations.

Maybe start with a funny one that sort of makes sense?

Type BAAA.