Well 4" on a unlimited is in that grey area, the same with 3" on two door. I guess it depends on lift because some people have the problems in this area and some don't. Most here will recommend the 2.5 for 2dr and 3.5 for the unlimited not to say you can't do a 3" or 4" but rather if you do and it ends up you are one of the few that ends up in failure then the board doesn't want you to think we were irresponsible in the recommendations. I mean you come here for sound advice I think, and the sound advice is giving what could happen if you do a 4" lift. Not just cheering someone that has never heard of the problem on in hopes they won't have a problem.
Fatal angle is not a absolute failure at 3" on 2 dr and 4" on 4 dr but rather it is a possible one when you hit a certain lift size and usually takes time before the problem occurs. And some are willing to take on the possibility and some would rather go safe and know for sure there will be no issues. Just know if you do it and don't change out the driveshaft with upgraded joints that you could break down some wheres you don't want to. Will it happen, no one can answer that for sure. But the great people on the board just offer advice to it's members so they know the possibilities.
I seen someone post the other day on this that they did a lift over the recommendations and seem offended because people gave the possibilities as if we are jealous or something. I think board members really just want to help give good advice, nothing more.