Already 4 and still not pooping in potty?

My four yr old daughter will not poop in the potty. She has the cute little girl underwear that she wears all day and keeps them dry. However, at the end of the day, there is always poop in them and on her bottom. She hates getting her bottom cleaned because it hurts from the poop having been on her all day. She would rather keep on the poopy underwear. It's so gross. I have tried everything I can think of. Bribery, grounding, spanking, etc. but nothing has worked. My boyfriend gets very upset with her and thinks that she should respond to the spanking and yelling and grounding. I wonder if it is something that we should leave alone and let her work out on her own. I mean, she has to get sick of it sooner or later, right? I am need of help as this is really stressful for all of us. Thank you for your input :)