1st, good on y'all for being able to take a CEL A/C deprived from you "brother in law" assuming BIL? PO from mine was a good guy...I still have some choice words for him when fixing things, ha. I'd be a little worried that could cause some issues at Christmas, ha.
I've got a 3.5" with some sag in the back. Working through lots of issues (sounds kind of similar). And I've been reading a ton here....and others would probably have more detail. Here's what I see which is similar:
I had a Drop Pitman Arm and just went to a Mopar factory one:
Before:
View attachment 184069
I took it off and also I followed the ZJ Tie Rod upgrade thread....and went ahead and got a brand new Drag Link, 2 Tie Rod Ends, and 2 new sleeves from Moog on Amazon. Painted them and put it all on. The drive is very, very different.
After:
View attachment 184070
I had tires off and set the toe in to 1/16th of an inch using the method with aluminum pipe from Home Depot that I found on this chat room.
The relationship of the Drag Link and Track Bar are not correct with a drop arm, the "Working Plane" that is drawn from mounting point to mounting point cross paths like this picture I drew of mine before:
View attachment 184076
I've read and now agree that 4" lift or less there's no need for this drop pitman arm. Someone that works on 50 rigs a year would know better when that number drives it (and I'm sure there's a way to make one work, going to factory seems from what I've read and now my personal experience to correct any issues).
Here's a pic after going back to factory:
View attachment 184077
I've seen the term "Parallel" to describe the plains.....someone jumped my chili saying it's not, whatever, it's pretty close and I haven't plotted it, whatever the description, you see the relationship in the pictures.
I just drove about 40 minutes on major highways and 40 back to a baseball fields here in Texas, and I can drive 75 with a light hand on the wheel. That includes some bump areas that I had to hold on tight, because I'd be at 65mph and the wheel would jerk. I put it off WAY too long, WAY too long.
Other than that, I changed out control arms to adjustable and front track bar adjustable. If you make those changes, I'm not sure if there's a huge need to go down from 4". I'm not sure I'm following the "why", you don't like the look, or other issue?
I'd spend my time getting the CEL to go away, and steering buttoned up, then make a list....and ask "why" on each one to yourself, if that's what you want. Unless you're looking for projects, there may be less that you need to do, no need to change things just for the sake of it.