Speaker driver sizes - pros and cons

To start with, the (3) 4 inch drivers will have about 1/3 of the surface area of a 12 inch driver. If you want surface area to be approximately the same, it will take (9) 4 inch drivers to be equivalent to a single 12 inch.

Thanks Erik, have posted the question there too.

Depends on what brand and composition of the drivers used as well as the xover and cabinet design.So size of the drivers are a very small part of a total design.

I've had a pair of Silverline Prelude small towers for years, and they're punchy, coherent, sweet sounding things. 2 3.75" (!) woofers and a 1" tweeter in each rear ported cab, all drivers are magnesium/aluminum including the tweeter...I recently replaced these (haven't sold 'em yet...hard to part with them) with Klipsch Heresy IIIs that sound remarkably similar in the sense that they're clean and coherent, a squat and fat 3 way with 2 titanium driven horns and 12" woofer. They don't play as low as the Preludes, but I've used 2 REL subs with both designs so no problemo, and I bought the Heresy IIIs simply to have more efficiency available. Utterly different designs, both do music amazingly well. Note that paper cones show up in some form or other in many high end designs (Wilson for example) as do soft dome tweeters, and they respond to what the designer wants to hear...my metal dome Preludes were never harsh or metalic sounding, and neither are the titanium drivers in the Heresys. 12" paper woofers...who knew?

Here are typical differences. These are not cast in stone.
Advantage of small driver:
better frequency extension in the treble region
smoother midrange response
tighter focus in imaging & soundstage
Easier to use in any size room (smaller)

Advantages of larger driver:
Moves more air for more impact in midrange and bottom frequencies, large rich images.
Better excursion limits for better power handling
Normally produces large rich soundstage
Normally higher sensitivity

Disadvantages of small driver:
Less power handling
Cannot portray the large orchestral experience as well
Limited Frequency response in bass
Normally less sensitivity

Disadvantages of larger driver:
Normally not near the treble frequency extension
Normally not as clean in the midrange
Normally requires a much larger cabinet
Beaming is often a problem