12" drivers don't have 12" cones, they are close to 9 3/4" Its called "sd" and its a part of the more complete sensitivity specification or sometimes called "spl" or splsens.
You need to look at the mfr spec, but 8's have about 26 square inches, so four of them would have about 100 square inches so you're off by a factor of twice the surface area or about 6dB... huge error!
12's about 75 square inches of area or 150 for a pair or about 50% more than four 8's.
If xmax is equal, then the 12's also have the displacement advantage down low. If the BL is equal then the 8's have a lower Q and can operate in a smaller box and have the power handling advantage.
If you have the space, then two 12's have higher value. 8's are really not an optical size for subwoofers so they tend to be nitch and nostalgic.