Got any ideas for 4 12's?

YIKES.  Is that intended to cover the rear window?  Yea, I think Rob's decapitation estimate is pretty right on.  What are you planning to load in that thing?Support the12volt.com

I think plans have slightly changed. Im loading 12 inch w3 d4's in it. After talking to a tech guy form jl for a while I decided i will prolly do all four facing the ground. He said that because I have little air volume in the truck it would help the subwoofers perform better. The plan was to make the box roughly 42 inches high. Yes it is tall. After going out to my truck and taking a look around I decide I wouldnt like that. In any case I was/am going to make a fiberglass overlay for the box. Essentially its function is just going to be to cover the part of the floor that doesnt have the carpet on it. WIth the way the plans are looking now though Ill be having a box with the demensions of 52in x 20.5 x 19.5 (l h w). Does firing the subs down with limited air space help?

Anyone?

Well, I have a 97 F-150 with a single 8" in it. When i first put it in i had it firing up into the back seat and it sucked bad. But when i flipped it over it kicked some butt. Seemed like it really brought out the lower end of it that i couldn't hear before. BUT, i have an 8", you will have 4 12's, big difference. With that many and that size, don't think it would matter too much. Its going to slam regardless!!