Does your truck have a block in the rear suspension right now? if so then you should get a spring lift for the back and from what I see their 4" kit comes with a block kit for the rear while the 6" kit comes with springs for the rear, though the site I was at had a bit more than a $200 difference between the two.
On the BDS-suspension site it tells you that the max recommended tire size is a 35x12.50(318/68/18) for the 4" lift and 36X13.5 (343/67/18) for the 6" lift. (metric mathematical equivalent in parenthesis)