Re: Main differences between 4x10 & 2x12?
12' speakers physically attempt to generate lower frequencies than 10' speakers. This results is a much tighter more defined bass response from 10' speakers. 12's will give you a much looser sound. I'm not sure there are many differences higher up in the audio spectrum, at least I'm not sure you'd really hear them even if they are happening in a scientific sense.
I know it sounds a tad obvious but it's still worth mentioning. 4x10's cover 40' of air while 2x12's only cover 24'. This means, to my ears at least, 10's in this configuration will sound much bigger. I think a 4x10 will often make a 2x12 sound boxy and small in comparison.
Hope this helps,
-Benja