Doc, Brandon is correct.
But you are far better off than guys who have it installed on a 6" lift (5" rear coil)
The way the airlift works is it compresses outwards so that the coils do not compress downwards
So essentially what the airbag covers is what will prevent it from sagging.
Guys running this on a 5" rear coil with heavy weight in the bed get max 1" sag, which is not bad if you have a slight rake.
That being said. I would estimate you to be 1/2" of sag if any at all.
Also, if you need pricing on the airlift, BDS kit, let me know and I will make sure you get Z member pricing