why do 4x12 cabs sound louder than what? a 1x12? seriously?
more speaker surface area also improves bass response. That's why bass cabs can use a bunch of 10" or 8" ( or even smaller ) speakers.
smaller speaker cones are also faster, so a big 15" speaker may have more bass than a 10", but is slower & less responsive. usually.
Throw a bunch of 10" speakers in & they're faster, and have the same low end as a 15"
speaker cabinet design is a whole other can of worms: ported, not ported, direct radiator, passive radiator, horn loaded, diimensions & size, shape etc all factor in