I'm not sure. But I think the success rate is higher on 4th and short (like inches, 1 yard, 2 yards) than it is for 4th and long (like 10 or 15 yards). Heh!
It SEEMS like 4th and short works about 40-50% of the time and 4th and long (when they have no choice to go for it because of the little time remaining) works less than 20% of the time.
So maybe about one third of the time overall. Total supposition on my part, though!