It depends on the sensitivity rating of the speaker.
For example, a certain speaker may have a 97db rating- which usually means that how loud it is with one watt at one meter with a SPL meter.
Other speaker models may rate at, say, 100db... and a 3db increase is considerable. It is @ the equivalent of raising the level of your guitar for a solo in a given mix.
Now back to the overall picture here: if your 2x12 cab has the exact same model speakers that the 4x12 cab you are comparing it to running it at the same output level on the amplifier, sure- you can say the 4x12 IS a louder cab.