Wow, I can't believe someone else is going through this. My daughter started doing the same thing about that age. She still occasionally does it but she's pretty much over it now. Isn't it frustrating! Her doctors believed she did it mainly because it's really the only thing SHE can control but I'm not sure if that's the real reason. I think it started after she had a really big, extremely painful movement. I think she was afraid to go after that but then turned into a control thing. I tried everything rewards, punishment, praise, stories, EVERYTHING! What helped us was I explained to her what poop was and why we NEED to go. I drew pictures of the body and digestive tract and showed her that poop was kind of like our bodys garbage and if we didn't get rid of it it would make us sick. When she would get a belly ache I told her that it was her body is telling her to go. I also gave her mineral oil (doctor recommended), yogurt, and high fiber foods like apples (not bananas), whole grains, and the occasional hot cocoa (chocolate is a natural laxative and the warmth gets the bowels moving). Don't put too much pressure on her. Let her take control of it, even if that means she has to help clean it up (believe me she won't like doing it for long). Also, make it seem like it's no big deal, giving it too much attention will only make her obsess about it and want to control it more. Last of all, relax and don't let it get the best of you. She will grow out of it and she still has time. Some kids aren't fully potty trained until 5-6, you should feel lucky she did it early.
P.S. Buy flushable wipes and always carry extra panties because with all that fiber and liquid she WILL have to go.
**If you have anymore questions definatly contact me, I know how frustrating it can get.