4 year old potty training regression

My 4 year old son has recently started having accidents.  He started potty training right before he turned 3, and he was fully trained within a couple of months.  He'd have an accident here and there, mostly at night, but it wasn't that bad.  I had a baby in June (2 1/2 months ago), but the accidents started before that.  It began as a little bit of leaking which I attributed to him not wanting to take time out to use the bathroom when he was playing.  Our daughter was born, and he still was doing okay--just leaking for a couple of days, and then he'd go back to using the potty for a week or so.  He seems to have adjusted to the baby very well.  He likes her very much.  Within the last month his accidents have gotten worse.  He's had many full out wetting accidents, and just this week he has started deficating in his underwear.  Just today he had done it 4 times!  We've tried so many things.  We've tried spending lots of time with him, giving him rewards when he uses the toilet, removal of some privileges, giving lots of hug & kisses, talking about it, etc.  He definitely gets a lot of attention since one of my biggest fears with adding a child to our family was our son feeling left out.  In two and a half months, he's got to do some really special things with just my husband and myself.  When I ask him why this is happening he tells me that he doesn't know how to use the potty.  I now feel like he hasn't adjusted to the baby as well as I thought since we've said before that the baby wears diapers because she doesn't know how to use the potty.  He also just started preschool last week.  Again, he seems like he's not bothered by it.  He runs off to see his teacher, he plays, he doesn't seem to be upset when I leave.  I'm just at my wit's end.  I don't want him to because the smelly kid that other kids make fun of because he's having accidents all the time.  I would appreciate any suggestions to help me help him overcome this problem.  Thank you in advance.

[91004]

I would take him to the doctor and have him checked out just to be on the safe side. My son is 5 now but when he was I guess 4 1/2 maybe less or more. He started having accidents, I thought the same as you well he just don't want to stop playing but even started making him try and sit on the toilet every hour to make sure and he would still do it , but his went away within 2-4 wks. He still goes at nite sometimes if he drinks past a certain time.