The percentages below are not exact, because I simplified things a quite a bit (basically assuming an infinite number of decks), but they're close enough for this I think. Also this is only if a 4 is showing, the 5 and 6 are left as an excercise for the reader.
A dealer with a 4 showing will end up with the following results:
Code:
17 12.1%
18 13.4%
19 11.8%
20 9.5%
21 11.2%
Bust 42.0%
Basically if you stand on 12 you'll win the 42% of the time the dealer busts and lose the other 58% of the time.
If you hit you really have seven things that can happen with your hand: <16, 17, 18, 19, 20, 21, or Bust. The chart below shows for each hand (1st column), how likely you are to get that hand (2nd column), how likely the hand is to win if you get it (3rd) and overall how likely you are to win with that hand (4th). If you sum up the 4th column you'll see that you only have a 38.4% chance to win if you hit on a 12 with a 4 showing (and a 4.5% chance to push).
Code:
Bust 30.8% 0.0% 0.0%
<16 30.8% 42.0% 12.9%
17 7.7% 42.0% 3.2%
18 7.7% 54.1% 4.2%
19 7.7% 67.5% 5.2%
20 7.7% 79.3% 6.1%
21 7.7% 88.8% 6.8%