I would say no way for a 4 yr old. We unfortunately had a really windy day. My goal was to see mickey under the water and that was it. He of course is the farthest white ball in the water. Swimming out wasn't bad because there is spots that are very shallow and we all could take a break (ds10, dd 12, ds 15, dh old). The 2 oldest kids gave up 1/2 way out there, leaky masks, the wind and the salt water and the distance. DS10 was a real trooper but finally gave up just about 10 strokes to mickey. He had had enough and went back. I kept going right to mickey and it took me about 1/2 hr to get back to shore. with the wind and the salt water in the eyes, it was really tough. I'm not that bad of a swimmer either. We unfortunately will never do snorkeling again. This was the 2nd time for our family and the 1st was swimming with the manatees in north Florida, screaming people who don't listen ruined that experience. My kids were the youngest and no one heard anything from them
So I would really recommend holding off the snorkeling until your child is a lot older. JMO