Logically, they couldn't do it. The Magic sails Saturdays. For it to do the 3 and 4 night cruises, it would have to sit overnight empty - since the 3 and 4 night cruises leave on Sundays and Thursdays. Now the Wonder would need to come in on a Thursday and sit til Saturday, being empty two nights.
OR
Everyone on a 7 night cruise would have to switch dates from Saturday to Saturday to Sunday to Sunday AND
Everyone on a 3 night cruise would have to switch from Thursday - Sunday to Saturday to Tuesday AND
Everyone on a 4 night cruise would have to switch from Sunday - Thursday to Tuesday to Saturday
Too, Too, Too, Too Much changes to even think about for 1 second.
It is true that the Magic did the 3 and 4 day cruises while the Wonder was in dry dock - but those were planned for and the days adjusted to fit the Wonder's timing.
After April 2004, anything is possible because they are not booking anything yet.