Don't get caught up in the 15" wheel thing and spindles. Worse case scenario is you use Belltech (or clones) and they push the wheel out a bit. All you have to do is trim the lower control arms for clearance. No big deal. I don't think anyone here has heard of any failures due to this. Guys and gals have been doing it for 30+ years on these trucks. And not once have I heard of any issues.
And unless you have a real good reason to only go 4", just do the 6" flip. Soooooo much less work. Those front hangers are what makes doing the 4" so difficult.
And then you will drive for a while thinking, you know....I could have gone that 2" lower and be ok. Then you have to redo a whole bunch more work, try and find your stock hangers in the garbage can, only to find out your kid actually took the trash out the one time you didn't want him to! Then you are trying to find replacements.
As
@618 Syndicate said, helper bags of you need it yo do truck stuff.
Not trying to poop on your parade, but a lot of people have gone the route you are planning, only to find out the hard way it wasn't the one they wanted to travel.
But either way....we are here to help!!!
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This is all the trimming required.