i went with rancho 4" and never regreted it but i paid $130 less than a 6" so i say go 6" unless you want to fit in the garage like i did.
NO you can run 35's leveled depending on lift liek fabtech you can run 36-37's depending on backspacing
if your leveled and get a 6 in you can make it a 8 8.5 or 9 and if you get the 4in you can make it a 6 6.5 or 7...all depending on your level