4 cylinders have no problem creating enough torque to crawl offroad. if you want to keep your jeep on stock tires, then yes the 4 cylinder is great, and gets good gas mileage. once you put some bigger tires on it, that's where the problem starts to arise. i drove my neighbors 4 cylinder yj with stock gears (4.10) and 33's. and it was pretty pitiful. i had to down shift going up hills at 45 to maintain speed.
if you are going to DD your jeep, or drive it on the road i would suggest just to get a 6 cylinder. if you're going to trailer it, 4 cylinder wont be too bad but with 37's you will need 4:1 gears in your t case