It's not right to say a 4X12 will be louder than a 2X12.
I have a G12T-75 loaded 4X12 and a V30 loaded 2X12.
The 2X12 is louder!!
Detune the a 2X12, ie remove one speaker, and it will be even louder!!
This is true; a 2X12 with only one speaker fitted is louder than if the two speakers are fitted!!
It it fair to say that when loaded with the extact same speakers, then a 4X12 will appear to be slighly louder by virtue of more air moving, but SPL wise it won't be.