I'm not sure you'd be able to do sub up/port back behind the third row...only because there wouldnt be enough space between the port and the back. I'd do the subs horizontally across with the port aimed up, all the way at the back of the box. You should be able to mount the 4kw's onto the front wall of the box. I'd do around 7.5 cu ft net, tuned at like 32 or 33 hz. You should be able to fit that without too much effort behind the third row. Give us some max external dimensions, I'll try and work something out for you.