"My son will be 4 years old soon. He does a great job peeing on the potty and he has gone poop before without any issues. Suddenly, he just doesn't want to use the potty. He wants to keep using his diapers. Should I be concerned about this at his age? I'm getting frustrated and I'm worried that the more I insist he use the potty the worse things get. Sometimes I am downright angry with him because he refuses and just goes and gets a diaper, poops or pees in it and then comes to me for a diaper change. He knows when he needs to go potty and he knows when he's already gone, too. What should I do?"
We don't always like to hear it, but sometimes potty training setbacks and failures have little to do with our toddlers and have a lot to do with our own decisions as parents. In this case, the problem is compound. There are really three possible issues to be addressed here and two of them have little to do with his readiness, skills or abilities. So, let's address these issues one at a time.
First Ask Yourself: Does Access to Diapers Make the Issue Worse?
First and foremost, I would say that you need to stop using diapers. Based on the information you've given me, there's one exception to this -- fear / pain -- and I'll explain that more in the last part of this answer.
Keeping diapers around can signal to your toddler that you're not that serious about potty training and you don't really expect him to use the potty. It tells him that he's got options. If he chooses, he can just go get a diaper and pee or poop there where he's more comfortable.
As long as the diapers are still there (and especially if they are directly accessible to him rather than in a high cabinet or other area he can't reach) then he's going to keep using them. It becomes even worse if he requests them and you give them to him. Move yet another notch if you say no at first and then give him the diaper after he begs, kicks or screams.
Ask yourself why you still keep the diapers around and address those issues as honestly as possible. Then, get rid of the diapers completely. If you keep them around because they're easier and you're afraid of all the accidents (this is a very common reason since we parents have enough messes to deal with and it can get tiring), know that he will just continue using them as long as you'll let him. Take a deep breath, expect that the next few weeks are going to be messy and jump right in with both feet (and plenty of paper towels and laundry detergent).
Also, be very careful about using disposable training pants like Pull-Ups. For a toddler who favors going in a diaper when he's already shown that he can use the potty successfully, using disposable training pants is a bit like going from the frying pan into the fire. You're likely to see a stall that's just as big as you're seeing now and clean-up with these pants can be more difficult than diapers. If you feel you must use disposable training pants, try to use them only at night and / or naps and stick to that rule without fail. The moment your child wakes up, change him into cloth training pants or underwear immediately. Also, make sure that you put the disposable training pants where they are accessible to adults only.