Try this....

COMPLETELY IGNORE IT!!! When he has an accident, don't give any
attention to it at all. Just say, "We have dirty pants... lets go
change those." That is it! Do not punish him. Don't ask him why.
Don't argue or make faces of disgust. Don't let him see you and dad
arguing about it. When he has accidents at school, just take his stuff
home and wash it without a word. It would also be very good to get his
teachers cooperation on this. Using shame is such a poor poor use of
discipline. It damages their spirit. I'm not saying you are doing
that at all, I was just wanting to point that out. Some parents feel
that they can embarrass their child into behaving, and that doesn't
work. It will just make it worse.

But here is the important part. When he goes in the toilet, make it a
very BIG DEAL!!! Praise him. Talk about it. Say things like,
"Doesn't it feel great not to have dirty pants!" "You are such a big
boy!" etc... Consider a reward, like taking a bike ride, going to the
park, renting a special movie, or going to the movies, etc...

The point here is this: Give attention to the behavior that you want.
Giving attention to the behavior you don't want, can create anxiety in
the child. An then you are just in a vicious cycle.

Or perhaps he is doing it for attention. He is wanting attention from
you or Dad, and he is finding that by going in his pants, he is getting
that, even if it is negative attention. Do you have a new baby at
home?

He may really have another problem going on, as some of the previous
posts mentions. Even if he does, it is not his fault. Just bringing
attention to it by punishing him or shaming him won't help.

And one more thing... give him a big hug & tell him you love him no
matter what!

Good Luck, Necole! He will get it. Keep in mind, he is not the only 4
year old who has had trouble potty training.