4" is that suspension lift height where things start getting more complex, especially where preventing drivetrain vibrations are concerned that result from the steepened rear driveshaft angle. I doubt that 1" transfer case drop they include with the kit will be enough to prevent drivetrain vibes that can't just be lived with since they can take out bearings, seals, and/or u-joints.
If you have an Unlimited, you'll probably be ok there due to the Unlimited's longer wheelbase and longer rear driveshaft which are less affected by suspension lift height. But if you have a standard TJ, you'll likely need more than just the 1" transfer case drop to eliminate the vibes. Adding a set of 1" taller motor mounts may be enough, but they may not be. 1" taller motor mounts help tilt the engine down a bit in the rear which reduces the steepened rear driveshaft angle caused by the suspension lift.
Better yet, install a SYE kit and longer CV driveshaft for around $500 which is the right
way to set up your drivetrain for that tall of a lift. Tom Wood has them for something just under $500 if I recall correctly and it's not difficult to install if you're handy with tools and are not afraid to get inside of the transfer case for a little minor swapping of parts that allow a longer rear CV driveshaft to be run.
If you stick with a 3" suspension lift, things will be a LOT simpler and less expensive to do since that height doesn't need a SYE and CV driveshaft, that transfer case drop kit will likely be all you need. Combining that 3" suspension lift with a 1" body lift would give you the same 4" of overall height gain without all the complexities a 4" suspension lift causes. Way simpler & less expensive.