D.D.
Need more info.
Was he previously trained, and has lost control, or was he never really potty trained?
What happens when he has an accident. Is he punished or scolded (which might make him try to hide it)? How is it handled?
Is it only at school or at home too?
Does your son communicate well with his teacher, or is he shy?
Does he have any developmental delays in general that this might be related to?
I could think of completely different ways to handle this, depending on the answers to the above questions.
ETA: Thanks for the extra info C.. I have 2 thoughts. One is that having a schedule with reminders is still really important at age 4. I almost never ask my 4 year old if he needs to go (he always says no). I tell him - hey buddy, it's been a while since you went potty. Please take a little break from playing and try to go.
My other thought is to ask if he recently had a growth spurt. One of my kids lost control after major growth spurts. I don't know why, but it was clearly developmental. He was 100% day and night potty trained at 2.5 years old. But with every major growth spurt, he would have accidents for about a month. Then he'd be fine again until the next major growth spurt. This stopped happening when he was 5. I was very frustrated at the time - not with him, but because I didn't know what to do. It turned out there wasn't anything I could do (and there wasn't anything he could do about it either), and he grew out of it.
To me - an accident or two a day at age 4 in an otherwise healthy kid - doesn't sound like something to worry about.
Good luck!