The 4" Rancho normally yields about 5" of lift.
I still think I may go with a 4.5" lift on mine some day, but if you think might ever look at yours and wish you had more lift, just go with 6".
For me, I think my truck would do most of what I would want as it sits with 35s and locker, but I want a 4.5" lift to put my front suspension angles back where they should be to get a better ride off road. I could care less if the truck next to mine is taller or bigger or whatever, but that's me.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT