Get the 8. When I picked up mine I had the choice and was going to just go to the 4 but figured what if I switch trucks? As it is I sit at a 2 inch drop all the time but that is with this truck. If I was pulling with the dodge 3500 I used to have I am betting I would need another inch or 2 of drop. Just my opinion but I would rather have the flexibility.
Btw congrats on the new trailer and on deciding on an Andersen. I pull the combo in my signature with one and love it. If you have any questions about it feel free to pm me.
Happy camping!