Hi Vanessa,
Welcome to the Moms Panel! A vacation should have a great mix of family time as well as adult activities - that is why a Disney Cruise is one of the best family vacations out there. The Youth Programming aboard the Disney Wonder offers supervised activities for children ages 3-12 - and for your four year old, I would recommend
Disney's Oceaneer Club. There is also
Disney's Oceaneer Lab, which offers some great scheduled activities - but I found it to have a lot more older children (8-12) which may be a bit intimidating for the little ones.
Once your child is registered, you can bring him in or out of the club at any time. When you drop him off, he will be signed in to the club, and will not be permitted to leave until you pick him up and sign him out. You are issued a pager, so in the event that he needs you or would like to leave the club - you will be paged to come get him.
The day you board, you can tour the Youth Programming facilities as a family. This is a great way to familiarize the entire family with the play spaces, and have any of those last-minute questions answered by the Youth Activities Counselors.
Good luck and have fun!