When my son was 3, he was completely pee-pee trained, but would not poop on the potty. We had been trying to offer sweet treats if he DID use the potty, but he never cared. But then we realized he got sweet things every now & then anyway, so it didn't matter to him. We didn't want him to feel punished, so we told him one morning that "since he was having a hard time pooping on the potty, it might because of the sweet things he eats. That they must be causing the problem. So we would have to keep all sweet things out of his diet to help his body learn to poop on the potty." He didn't get much junk food, so this included ALL sweets...cereal, juice, fruit, pb&j, anything that tasted sweet. He pooped that day! The next time he went in his pants, "Oops, must be those sweets again...no more until you've stayed clean & dry for 3 days." So he stayed clean & dry for 3 days and got his sweets back. That was it. He was trained. It was very important to him to have his sweet stuff!!!
My daughter, on the other hand, was very laid back & hard to motivate. So that didn't work with her. She held out for 3 weeks with no sweets. I finally gave up, but of course I couldn't tell her that. So we just told her that the sweets must not have been the problem for her, so we would look for another solution. That's when I read an article that suggested the following:
Again, you don't want to punish them. We train them to poop & pee their pants from birth. It's not their fault. So I told her that "because she was so big (also 4), that baby wipes were too small for her. The only thing I had that could clean her big ol' bottom, without making a big mess in the house was the water hose outside." I was very calm. I didn't use it as a punishment. I just told her I was sorry if the water was cold...but this was what we'd have to do since she was so big...unless she wanted to use the potty instead. After 2 hosings, she was cured!
You could have them try these things. Some people may think it sounds cruel. But, like I said...I made sure to be very calm & loving. It was not punishment...just news ways of doing things "because they were so big." it's worth a shot. They've only got a month or so!! Good luck to them!!!
C.