You can find a spark plug wiring diagram for a 1998 Jeep Cherokee in the vehicle owner's manual. You can also find the diagram at various auto stores.
1998 Jeep Cherokee 2WD 4.0L MFI 6cylThe Spark Plug Gap.035 (In thousandths of an inch)
I beleve it is stamped right on the exaust manafold.. on the drivers side
I know it is .035 for a 1996 4.0L
0.9 mm / .035 inch owners manual
I was looking at the 1998 Mercury Villager Owner Guide ( and one of the Ford websites ) and they both show the spark plug gap as ( .034 inch )
If it has the 4.0 in it the spark plug gap is 0.035
Spark plug knock on a 1998 Chevrolet pickup is an indicator that the ignition timing is correct. By adjusting the timing, the knock can be eliminated.