The keycodes are pretty easy to guess. Regular users will only ever enter the correct codes so if you look carefully at the keypad you can see which digits have been pressed. Most likely there will only be 4 of the 10 possible digits used. I've actually encountered keypads where the order didn't actually matter so you could get in in seconds.
Combine that with the assumption that the average consumer probably used a date of some significance for their 4-digit code and in the US you can guess that 0 or 1 was first, 0, 1, 2 or 3 was the third position, leaving you only to guess the second and fourth position.
There are only 24 possible combinations of a 4 digit code anyway (assuming 4 different digits). I doubt the devices do a lockout on the wrong code, so it wouldn't take that long to hack.
>I've actually encountered keypads where the order didn't actually matter
If the order really doesn't matter, then that counts combinations twice (1234, 1243..). There are 10000 possible 4-digit codes, 5040 permutations, and actually 210 combinations.
