Fridays are less busy
If you book the dinner and they change it, yours will be changed over and the price set at whatever they decide.
A 4 yr old who likes SW will enjoy a good portion of the events. The wandering characters, the parade, the fireworks, Darth Mall, he may like to meet the characters too. He might also like jedi training and the padawan mind challenge (it is cute even if the kids have no idea of the answers). Trading with the Jawas could be fun for him too.
I dont recall when they announced the meals. It wasn't too far in advance if I recall. They will likely announce much farther in advance this time.