I had '96 and '99 jeeps with the 4.0 and they were very reliable and lasted a long time (bit of a gas hog though). Those engines have the reputation of being bullet-proof and held in high regard. here's a little history of that motor:

http://www.fourwheeler.com/how-to/engine/154-0604-jeeps-kick-[censored]-engine-history-4-liter/

if you want to read the article, replace the **** in the url with the obvious 3-letter word that goes there. I guess the software filtered out a legitimate english word...