IM running 4 15s, subs up port back in a Navigator on ONE 3500d.1. Don't know how thats not similar. Less cone and power at that. Hair tricks are no problem at all. I could do hair tricks with 2 L7 15s and only two ZX1000.1s in the same vehicle. As I said before the enclosure will make or brake you. If you have the larger newer expedition you "might" be able to fit 4 18" subs up port back with 2nd row still in, doubt it, but walled or going to 15s would be even better.
I also designed a box for 2 18s in a 4 Runner on 1800rms that can trick all day long. Subs up port back. Again Enclosure WILL make or brake you. If you cram 4 18s with low net and port area its not going to do half as good as doing an OPTIMAL enclosure to what FITS the vehicle.
4 18s and 5k power is more then enough if designed correctly ie enclosure that works well with the equipment or could also be the biggest waist. Sometimes less is more.