you don't have enough room in an explorer with out drastic measures for 4 tempest your talking 16 cubes plus port 20 cubes of box easy for these subs to perform optimal spl and sound there best IMO , you will have to ditch the rear seats. however there is a cat running around here with a similar set up in a single cab pickup (cut through) that does 150 @ the head rest
with that said i would go with 4 tempest if i had the room you could power all 4 with 1000 watts and it would be very very loud much louder then 2 Brahmas in equal box's
you got 20 cubes go for it
i have 2 in a dakota and it is pretty sick