There's not much limitation at all with a 4 inch. As for Glocks which I use in IDPA, I find the balance and the quicker draw of the G19 actually helps vs. G17 or G34. The sight radius and lower recoil help in the larger guns. It's a trade off. Why will you shoot IDPA? To win your State Championship or to get better with your defensive handgun. If it's the latter, shoot the G19 and don't worry about winning.
I've seen people win with a G19 though.