Okay, now can you please give me a stronger hint?
4 x 4 is not sixteen; it's just four times four
Yeah, uh... I still got nothing. Tried entering 4x4 into the doors (nothing happened) and tried to type the word four into the doors, too (didn't work).
It's a 4 digit code.
4x4 tells you all the digits.
I finally got it!
