4 15's have 4x the cone area of 2 12's. 5mm of excursion from 4 15's is like 2 12's moving 20mm. 4x power usually doubles excursion assuming no compression. So you really need 8x the actual power on the 12's to match the 15's given no compression. Even then your approachign system limits either via port compression, power compression, BL compression, or just mechanical travel in general. In the right setup where space is an issue 12's would make a better choice, but all things equal it's not even remotely close.