Yes, if you stick with just the complete 4" kit it will be okay...that's why it's a complete kit.
When you start adding to and changing the components of the kit, they stop working well together. If you try to go HIGHER than the kit is designed, like adding another 2" of spacers or height in some way, everything is out of alignment again. The problem is, you are already so far from stock height that other things start happening, like the rear driveshaft length and clearing the gas tank.
Think about it this way, once you put the complete 4" SL kit on, almost everything is back to stock, OEM geometry. NOW, try to build a 6" lift kit from scratch with OEM geometry- how many parts will you need to replace? This is why you'd have to immediately replace most of the SL parts if you are going to go HIGHER- all the rear control arms are now too short, your front upper arms are now too short, all your sway bar links are now too short.....see where I'm going with this? Yes, SOME of the SL parts will help, like the extended knuckles, front diff drop and the front lower control arm drop, possibly the spacers (though, it'd be much better and safer to do taller springs and struts at that height), but otherwise, almost everything else is too short to go higher in the same fashion that the OEM parts were too short for the SL kit.