yeah those 4 cylinders SUCK. my brothers first jeep had one, absolutely no power and only got 15 mpg with i think 31's on it. he got rid of it, got the itch again, and bought another one with the 4.2l in it. it's not that much greater. turnin 33's, he hasn't put it in 5th gear since he bought it, can use 4th on the roads on flat, but has to hit 3rd for any slight incline :-( but it's got a really long stroke, max torque is at 1800 rpm's... and the heads suck on them, the ports make a 90 degree turn. whoever came up with that design was a moron, but a 4.0 head bolts up, has better port design and smaller combustion chamber (upping compression to something like 10.5:1), supposedly a 40 hp increase overall, so we're thinkin of goin that route cause 4.56 gears are like $400.....
also, you can throw a 4.2l crank in a 4.0 block and it turns out to be a 4.6l torque monster. the 4.2l has a smaller bore but bigger stroke than the 4.0, so when you combine the big stroke of the 4.2 with the big bore of the 4.0 it makes 4.6 total displacement. supposed to be an insane motor.