Well, with a 4" suspension lift, you either have a slip yoke eliminator (SYE) installed or you have a transfer case drop installed. Do you see spacers between your transfercase skid and your frame? That would be a tcase drop.
Stock, the ouput of your tcase is a slip yoke. The end of the driveshaft slides in and out of the tcase tailhousing. This is to allow the driveshaft to lengthen and shorten based on what your rear axle is doing in relation to the rest of the Jeep.
There's two problems with this setup. First is the drivetrain angle. Once you lift the Jeep, the only way to correct the driveshaft angle is to drop the tcase a little less than 2", which changes the upper angle. The angle on the pinion must be parallel so you must shim the rear axle up 2 degrees to match the new tcase output angle. The second problem with this setup is that if for some reason you have a failure that requires you to remove your rear driveshaft, your fluid will pour out the hole the driveshaft was in.
Dropping the tcase and shimming the rear axle is not the ideal situation, but it works for lifts up to about 4". You can plug the hole with just about anything to get you home in front wheel drive if you have to also.
The ideal situation is to eliminate the slip yoke with one of the half dozen kits available so that the hole is permanently plugged. You will have to install a driveshaft that has the slip built into the shaft. The SYE will additionally allow you to run a CV type shaft. With a CV shaft you basically have two u-joints at the top of the shaft. They split the angle by themselves. So you rotate the pinion up until it is within 2 degrees of the driveshaft angle. Takes mucho stress off of the pinion joint. Also, you can run much steeper angles this way. If you go over 4", you pretty much have to do this mod.