I'll chime in as another one who never drove the 4-cyl GAS liberty (the 4-cyl diesel KJ is AWESOME though...and it doesn't use ANY gas

). I have, however, had experience with a 4-cyl Ford Ranger. It was a great truck, but the 4-cyl ended up giving me the same gas mileage as my friend's 6-cyl ranger. As has been stated, the 4-cyl needs to work harder to push a truck through the air and over the hills.
So, if you're looking for a 4 for the gas savings...don't, it's not worth it.
If you want a 4 due to lower initial cost...do. That's why I had the 4-cyl Ranger and it lasted a long time and did everything I asked of it. I would, however, strongly suggest only getting a 4-cyl mated to a manual tranny. 4-cyl engines and automatic trannies should be an illegal combination.
So, if your parents don't want to shift...there's one more thing leading toward a 6-cyl.
Just for informational purposes: I don't know about the earlier models, but the 4-cyl gas was available in both 2WD and 4WD during it's last ('05) run. It was also available with the auto or manual tranny.
Good luck on your search.