A 6" lift will keep the correct suspension and driveline geometry (diff drop, sway bar links, etc.)
You would also have to do something with the rear if you did the 4" kit too. Most lifts will leave your truck level so you'd be nose high with the leveling kit
You can run a level (strut spacer style or leveling struts) with a lift although pretty much every manufacturer says not too (liability and saftey)
Some kits come with preload spacers that have to be removed