You'll most likely need a SYE. Each vehicle is a bit different and some Jeepers have come up with a work around but with a 4" lift, the SYE is the most common cure for the issues that might arise.

Jeep Wranglers typically have a short wheelbase. This contributes to steep driveshaft angles. When you install a spring lift, it increases the steepness of the driveshaft angle. It's easy to lift a Wrangler far enough that the extreme angle causes vibrations. A 2 1/2" lift is generally considered the max a person can lift a Wrangler without a Slip Yoke Eliminator.
What a SYE does is replace the stock tailshaft housing of the transfer case with a shorter housing. This also requires replacing the TC output shaft. With a shorter tailshaft. You need a longer driveshaft, thus reducing the angle of the driveshaft. The longer driveshaft has an included slip yoke and a double cardan joint. There are typically custom made for the particular Jeep that's getting the SYE. Tom Woods is a good source for a custom drive shaft. You'll also have to shim the rear axle at the springs to raise the pinion shaft angle.

Google Tom Woods Driveshafts for a better description of what needs to be done.

Good Luck, L.M.

 

Here's a good writeup.
Jeep Wrangler Slip Yoke Eliminators Explained (extremeterrain.com)

Advance Adapters or JB conversions are the most popular kits. 4" Suspension lifts almost make them mandatory. Forget the almost, they are mandatory to keep from damaging other components

 

We are adding a RE 2.5" lift during our YJ rebuild and considering a SYE kit. I'd be interested in thoughts here.

Any pros & cons between the brands? I see that Extreme Terrain has Red Rock, Rugged Ridge and Teraflex kits.

 

I think most of SYE brands come out of the same factory in china. The only stand out kit that is built better is JB Conversions you can see comparison pictures of the JB Conversions kit vs the others. I have bought and used 2 JB Conversions SYE's and i wouldn't waste my money on anything less.

 

SYE fixes several problems. The first problem it solves it the weak main shaft in the NP231 that are known to break when you wheel hard. The replacement shaft is much larger diameter and the size of a NP241 main shaft. And it has a fixed yoke so if you happen to break a driveshaft you can drive in front wheel drive if needed.

SYE also makes the overall transfer case length shorter allowing for a longer driveshaft. Short wheelbase jeeps suffer from bad driveline angles even from the factory because the driveshaft is so short. Once you lift them you have to drop the transfer case and shim the rear axle to allow for proper angles.

The SYE allows you to run a longer double cardan shaft so you do not need to lower the transfer case. So you can have more clearance and you can run a longer double cardan shaft that when properly set up will not have any vibrations.

 

You need one with 4”, may not be right away but like me, you will end losing your driveshaft and all the transfer case fluid resulting in being stuck waiting for a tow. Like Agalloch mentioned, with the SYE, you can drive away in front wheel drive WITH fluid in your case if you lose a driveshaft.