Seeking wisdom from those who have installed a lift

RustyRatRod said
....and 4" minimum MEANS 4" minimum. NOT 3.5" used with 2 x 4s for framing up. You have to watch it, because a lot of contractors will call a 3.5" pout a 4" pour when that extra 1/2" matters.

Yep, then tossing loose dirt in to fill the low spots. Compaction is a Nice thing also