Kinda feel there should be a hint for the safe somewhere, but... Well, I did find it by accident without ANY hints...
I mean, it did make me laugh that literally the first code I randomly tried worked. But I've played a lot of adventure games, and so have so many other people. It took me a good while before I decided to just try something. Though, I did also think for a moment, "...Is this thing broken and any 4 digits work?"