Isn't the 4" really designed to get more 1911 guys into the poly world? If you're carrying a 1911, then you may want the 4". Otherwise, it's really not going to make a nickels worth of difference. Barrel length isn't the tough part of concealment.
Do you want to buy different holsters, or do you want to utilize the same one you have?
Do you want something different to play with? You already have a 3.3, why not pick up the 4" to see for yourself.
Me, I'd get the 3.3. I love my .45 xds and I can't imagine that the tiny barrel length difference is going to make me shoot in perceptibly better in a real life situation. Maybe if I was doing CCW shooting competitions....
In short, get either one, you'll love it. But if you get the 4" you're going to have to buy another holster, that's what would keep me from doing it.