There really aren't that many differences between the two. They offer the three broadway type shows (currently Toy Story, Golden Mickey's - not sure if they've stopped that yet and soon to be Villians, and Disney Dreams). I can't remember what they offer on the extra night for the 4th (possibly a movie in the Walt theatre?). You still have the POC party with the fireworks.
I don't believe brunch is offered on the 3-day at all since there are no at sea days and I believe this only happens on sea days. There is no "tea" on either 3 or 4 days. Mostly the time factor as you may have the same amount of people on either cruise and there is less time bookable at the spa, less time bookable for Palo for dinner, etc.
The differences are more apparent between the 3/4 day and the 7+ day cruises.