4 Year olds potty accidents only at home

My son is 4.  He has been day time potty trained since he was 3.  He continues to wear a pull up at night.

Our main problem is he has wetting accidents almost daily, but only at home.  Often more than once a day.  He goes to pre-K 5 days a week (3 days until noon and 2 days until 2:15).  He never has an accident there.  He also never has an accident when we are out or if he is playing at a friends house.  This summer we went on vacation for 3 weeks and that whole time he only had 3 accidents.  He always makes it to the bathroom for bowel movements.  His ability to make it to the bathroom when he is away from home indicates to me he is aware of when he needs to go and how often.  So why is he constantly wetting at home?

As encouragement I allow him to get a jelly bean every time he goes to the bathroom at home.  I have tried a reward system where he had to go 5 days with out an accident and he would get a special truck.  The day after he earned the truck he wet his pants 3 times.  I took the truck away for 3 days.  ( one day for each wetting).  The day after he earned the truck back he wet himself again so I took it away again.  I have also taken away special things like his blanket and teddy bear with out results.  I have tried using tickets.  Each time he wet he would get a ticket if he got 3 tickets in a day he would have to go to bed 1 hour early with out a story.  After a week of going to be early I gave up.

When he has an accident I tell him to go to the bathroom, finish going potty and then change his underwear and pants. (which he can do himself.)  Sometimes he changes without even telling me.  

Other than this I have no decipline problems with him.  He has two older sisiters who were potty trained just after their 2nd birthday and night time trained by 3 with hardly any accidents.

[Kevin Kennedy, Ph.D.]

I'd stick with the reward system for now and this behavior will likely wane. What has he told you about the pattern? Try not to display too much 'investment' in the outocme, but also make it clear that you want him to use the toilet. The reward approach vs a punishment approach will support him better and make it less likely that you'll get into struggles about this.

[Amre]

He really can't give me an explanation about why he can make it to the bathroom everywhere except at home.  I know he gets very intense and involved in his play at home and probably doesn't want to take the time to go.  But I'm sure he gets involved in play at friends houses and school too.  I guess when he is at home he just feels in a comfort zone and knows there is always more clothes to change into.  With regards to the reward approach should I just concentrate on small rewards like the jelly beans?  I think we made a mistake with the truck reward.  It was a $50.00 remote control truck.  He wanted it so much so I thought for sure when he got it he would work hard to stay dry.  But like I said the day after he got it he wet 3 times.  I can not see letting him to continue to play with something like that after so many accidents.  Shouldn't he learn that there are consequences to his actions?  Should we just put the truck totally away until the accidents fully stop or should I continue to use it as an incentive?