as long as there is nothing seriously wrong with it right now, i would just use it till something breaks, the 4.0 is an excellent motor to start with, anything you do to it isnt really going to gain you lots of power for little money.
when you really want to make it scream, look into a stroker kit for the motor, but i doubt if i would bother myself.
the only real weak issue on the 4.0 is the rear main oil seal, and if you do that, pull the tranny, do the clutch/torque converter, and check the flywheel.
1998 XJ on 37's.JK Rubi axles with chromo shafts and ctm's, 4:1 transfer case, etc etc etc.The Cherylkee....
2003 Range Rover HSE, 4.4 Quad Cam.
1998 Range Rover 4.6 HSE
2000 4.7 4WD WJ with a rebuilt motor.
4.3/700r4/new frame buildup (YJ)....
Years to gain the hands on experience, and offer it freely.
Minutes to gain the knowledge from the internet, and quote it just as freely.