I'm a believer in Jesus, but I'm not buying this. I'm guessing it started out with him having a dream based on what he's been taught about Heaven and Jesus. Then his parents got all excited and questioned him. It's very easy to make a child "remember" things that didn't happen, even if you don't mean to do it, just by the way you word your questions. A four year old will want to please you, and give you the answers you're looking for. Then, as time went on, he told the story so many times that he really thinks it happened.
That's just my guess, though. I don't claim to know everything, so I'm not willing to say "this never happened." I just highly doubt this story.