I can't say for sure on how much height you will get. BUT if you don't crank up the preload too much, the truck will ride great on road, and you will really notice it off road. The newer coils probably gave you 1" of lift? so you're at 3" over stock? You may be able to add a 1" spacer from PRG to the DR coils and keep the height and also keep the nice ride.
Contacting ICON would be your best bet to find out how much height they say you can get without trashing the ride. I just went through the same thing with my FOX coils, 1" spacer was the best for me.