I'm probably one of the few who did upgrade from a 4 to a 4S. I've found the speaker to be a bit louder, as well as the earspeaker (?) to be significantly louder. With the 4, using the phone up to my ear, I usually had the volume at 100% to hear decently. Now with the 4S, I can have it set much lower and still can hear better.
The speaker, whether on speaker phone or playing music, does seem to be louder as well, though I haven't done any side-by-side comparison.