4 Year Old and Unexplained Self-Wetting

My son is 4-1/2 years old and has in the last 6 months developed a peculiar habit.  He sometimes wets the bed while asleep, which we assume is because he can't wake up, but recently he has started doing it while awake as well.  We will change his clothes and bedding and put him back to bed after the first incident, and then he will urinate on himself again while completely awake.  One night, he did it four times, getting us up each time to change his pajamas and his sheets.  

My son does not self-wet during the day, and we are sure that he is fully awake for the subsequent wettings.  When we ask him why he didn't get up and go to the bathroom, all he can say is "I don't know."  Since he does not have this problem during the day and is fully awake, could he be urinating on himself for attention?  Or could there be an underlying medical cause?  We have asked him if his stomach or genitals hurt, and they do not appear to be hurting him.

[Niquea3]

When I was pregnant with my second son my first one grew very jealous. He was 4 1/2 at that time and started acting out in ways that I knew for him were abnormal. Not only was I having the same problem your dealing with but he would claim he was having bad dreams and wake up every night to get in bed with me. Looking back I know it had a lot to do with the growing attention around the new baby. Are you pregnant or have you and your spouse been really busy with life/work and not giving him as much attention as you think you were before? It could also be an act of regression as another reason for attention. Put him in pull-ups at night to not only show him that its for protection but that (and only if you feel its for attention and not anything medical) because those who aren't big boys yet have to wear them like babies. Once I tell my boys their doing something that big boys wouldn't do it makes them straighten up because they want to grow up and be big so bad.
Hopefully it is all for other reasons mentioned and not medical. I would also talk to your pediatrician and get feedback from them.