You can, in most any case, go down on any kit. What you can’t do is go up on most.
I wouldn’t be worried at all about going down to 4”, although I personally think 4.5-5” is where you are going to want to be on a modest tire. Like say a -12 wheel and a 35 or something. If you went 0 through +18 on the wheel then maybe 4” would be better.
FWIW, there are supposed to be real 4” kits by the end of the year allegedly. Superlift is doing their kit right now, you should check with them and see what’s going on with that one. It likely won’t cost too much more than RC and the quality will be as good as anyone else.