Except though, how often do you only press the four digits of your pin. When you make a deposit of $10 or more you need to press at least 4 digits, the dollars and cents. So now you've pressed 8 numbers, and someone has to figure out which of the 8 buttons are for the pin #.
After 3 failed attempts the machine eats the card, and if it's retail the cars gets disabled.
So even best case scenario of having 24 combinations, you won't make it past 3 attempts.