I have a 2.5" RE lift with no modification except a t-case drop, but the RE literature recommends a driveline correction for anything 4" and above. It doesn't say "required" for 4", but it does say "recommended", so I guess it depends on you're setup. Others with a 4" lift would be better able to advise you.
It all depends on the vehicle and lift. I had a 4" RC, and all I got was 4". I have heard that with the RE you may net closer to 5". I had a couple vibs with my RC, but I dealt with it for 2 years until I installed a SYE. It depends on the lift, vehicle and your budget. If you don't want to do a SYE, you could try the t-case drop and maybe some shims, but if you do end up somewhere around 5" you may just want to do it right and get a SYE.
I just had this problem 2 weeks ago lol. I installed a 1" BL and MML (MML was supposed to help with the driveline angles), a few weeks later I installed my RE 4" lift which my jeep already had 1" lift shackles after getting the TC drop and the rest of the kit bolted in I took it out for a test run and it popped and vibrated bad, so bad I didn't drive it until I got my SYE and CV shaft on. I did take the 1" lift shackles off and put 5/8" MORE boomerang shackles on and that did help the vibes. I have less than 300 in my SYE and CV shaft. I bought a JB coversions SYE kit which was $213 shipped, an XJ Front shaft cut and balanced $84 cheapest SYE kit I could find was over $400 and I didint' like the kit so I built my own lol hope this helps.
Here it is on 35's my old 1" lift shackles and TJ flares
97 Cherokee Country, all stock.... For now
[B]SOLD[/B] 1993 Wrangler, 4 cylinder with a bored out TB spacer intake manifold and a 4.0 HO TB, 5 spd, herculined tub, 1in Performance Acessories BL, JKS 1in BMML, TJ Flares, Rubicon express 4in lift, M.O.R.E. 5/8" Boomerang Shackles, and 35x12.50x15in BFG Mud Terrains
A shackle that provides 1" suspension lift will change your pinion by 2 degrees. (use that high school match to figure out the circumference of the circle created by your leaf spring).
Vibrations will depending on your pinion angle vs. transfer case as well as the total angle of your driveshaft. The rule is that the angle at the pinion and transfer case should be the same, however a ujoint only works "vibration free" in a very small angle. With a 4" lift, the angle of the driveshaft is becoming extreme for the ujoints and you will get vibrations due to the overall angle, not from mis-matched angles.
On my Jeep, I have 3.5" springs and shackles that provide 7/8th" lift. I had vibrations that I could only solve by pointing the pinion up. So my transfercase and pinion don't match exactly, rather the pinion is slightly more. It was a tradeoff for the least amount of vibration. The downside is my front driveshaft vibrates at higher speeds.
So the only choice for me is to put in a SYE as it will allow the me to set the Transfercase up so the front driveshaft is vibration free, and the rear can be pointed right at the transfer case.