I'm just going to chime in and add I think the age restriction is there for a reason. We took our twins who were almost 8 years old on our last cruise. Unfortunately, while we were about as far out as you can get in the snorkel zone, out of nowhere a storm came through. The sky went from sunny to dark in minutes, the waves became very rough going in the opposite direction as the path to the shore (you can't get straight to shore from parts of the snorkel area because there is a rope--you have to go around to the side). By the time we realized what was happening and started to turn back, the water was pushing us backward in between strokes, and the twins couldn't swim forward at all. Luckily DH and I are strong swimmers because we had each took a twin and let them "ride" on us the whole way back. My muscles were screaming when we got to shore though!
I'm not saying that is typical or anything, but it definitely gives me pause about taking a 4-year old out. Pool snorkeling and ocean snorkeling are not the same thing. In fairness, we were there at the tail end of hurricane season in November. When we went the prior year in August, the water was smooth as glass.