Why won't my almost four-year old potty train?

She will be four in April and we would like her to start preschool this coming school year, but they will not accept her if she isn't completely out of pull-ups by then. We've been working on potty training for awhile now. She understands the concept and has used the potty many times (small one her size and the big one). We've always made a big deal about it to keep her motivated. We clap and cheer and give her treats. We tell her what a big girl she is and that if she keeps doing well she will get to go to school with her older cousin. She was doing really well for awhile. She was peeing in the potty on a consistent basis and we were working on pooping. But now it's like she doesn't care anymore. She knows when she's had an accident, but she's not embarrassed when it happens. She doesn't care. She will come and tell us very nonchalantly that she's either wet or she pooped. She knows when she has to go because she has told us in the past. I'm starting to think that she is just being lazy. That it is easier for her to keep playing and just go. We've tried just putting her in underwear, but that doesn't seem to make a difference. She will still wet her underwear and expect to be changed. One time we were sitting on the couch and I asked her if she needed to go. She told me no because she already went. Then she showed me that she had wet herself and the couch. It was no big deal to her. She literally shrugged and then went and got clean pants and a new diaper for herself. She refuses to change herself and I refuse to leave her in wet clothes. I don't understand why she is doing this now when she was doing so well before. Any helpful suggestions?