my 4yr old survives on 3-4 hrs sleep a night

i need help!my son is 4 next week and he only has between 3 and 4 hours a night. He goes to bed fine, falls straight asleep at 7pm but within an hour hes awake for a wee(despite having one at bedtime). He will then go back to sleep for 2-3 hours and from then on he constantly comes out of bed. As soon as i get back into bed hes in my room again!It goes on for the rest of the night and i'm so tired,i take him straight back to bed each time, his excuse for getting out is different each time, but none of them are valid reasons. He has a bedtime bath, a warm drink and a good bedtime routine but its like he has that one hour of deep sleep and then he feels he doesnt need any more. I've taken him to the doctor to be checked over but they say he is fine but he is so grumpy the next day(through lack of sleep) and i'm at my wits end.he wont have a nap and yet is dead on his feet by bedtime. He starts school fulltime soon and i'm dreading what he will be like if we dont sort this problem out!i've tried reward charts and special treats and he understands that he shouldnt be getting out of bed but its made no difference.

[mammo]

Check his diet and make sure he isn't getting too much sugar or any carbonated drinks with caffeine and sugar.  You may want to try putting him to bed a little later, and a lot of kids are not napping any more at this age.  If this continues, I would get a second opinion on the sleeping.  It doesn't sound like he really has any reason for getting out of bed other than not being tired. But I would be curious to know what reasons he gives you, there may be a clue there as to why he's not staying in bed.  Is he scared, does he just want to be with you, etc.  Let us know, maybe we can see something in his reasons.