Jodi, I have had this problem with my son for about 8 months now. He was potty trained at 3 and did a great job! He practically trained himself. He will be 4 in June and he also will not go Poop in the toilet!!! We were making him try to sit every day on the toilet but he would not go. So then he would get constipated because he would hold it in and no ever let it out. My pediatrician said to use a timer, calendar, rewards, etc. but none of these were a success. He would only go in his diaper. He eventually ended up in the hospital with a impacted colon from holding his poop for so long. We decided then, to let him go poop in his diaper, because at least he will then poop!! So, yes we are changing poopey diapers of a 4 year old also. My husband and I have decided that hopefully he will grow out of this. I have told him that he will not be able to go to kindergarten if he is still in a diaper for pooping. We have tried everything and he just won't do it. My doctor said that this is a common problem with BOYS! My husband as a child had this same problem.
My son also has a definate position for his pooping. I call it the "downward dog" position. Are you familiar with that pose in Yoga? You are kind of in a hands and feet on the floor with your bum up in the air. Why that works for him I don't know. We even bought a new toitet seat that has a extra seat inside of the lid that flips down with a smaller opening for smaller bums. I thought that he might think that was fun and new, and would then try going on the toilet. NOPE!! So we just wait. I think that they will pobably outgrow this problem. My docotr assures me. My girlfriend has a little boy who is 3 1/2 and has this problem toooooo! What is a mother to do?
Hang in there, hopefully they will get tired of poop in thier pants soooooon!
R. K.