Hey there! Here's my opinion, for what it's worth: we did the med cruise add-on in 2011, and it was my favorite vacation EVER. We loved everything about
ABD, and they filled the trip with days full of good times. That said, I would not take a 4-year-old, even an "advanced" 4-year-old, on an ABD. The days were very full, lots of riding on busses, walking, crowds in the cities, and most of the activities were not ones that a child that age would get anything out of. We have now done 4 ABDs, and I think the youngest kid in any of our groups was 8, which I think is a good age. For the most part, the majority of kids in every group have been in the 9-12 age range, and I think a lot of the activities are geared perfectly that way. We just got home from Costa Rica, and all of the kids in our group ranged from 9-12, with one 14 year old. They had a blast together! I would recommend saving your ABD experience for when your kids are a bit older, then by all means go for it! It's worth every penny, but probably not if you're 4...