I think 4 tens in your rex would hurt the pressure flow. 3 tens would gain i bet but 4 tens would be slim to nill, givin the same ammount of power. How much do you gain from 1 sub to 2 subs with the same power?

 

about 1.5 db's.

 

I roughly figured out 3subs would be aproximently 0.6 to 0.4db and the 4th sub about 0.1 to 0.2db, assuming the pressure flows equally around the boxes.