What is the oil capacity for a 1989 Jeep Wrangler 4.2L 4cyl engine?

Your information is conflicting. The 4.2L is a 6 cylinder engine. 4 cylinder YJ's all have a 2.5L engine, so I'm not sure which you have, but remembering the capacities is easy. According to the Chilton manual, it's 4 qts. for a 4 cylinder, 6 qts. for a six cylinder. Both volumes include the filter. Always be careful and don't overfill. To be safe, I always add 4 qts. to my 4.2L, then add 1/2 qt at a time checking the dipstick after each addition.