itd be best if you could use the lor. i mean 4 years of working for a place should render a decent letter.. like the other posters have said, is there no way you can somehow see this letter?
how many negatives do you really think there can be? i mean is she spiteful that she would say she would gladly do it and then write bad stuff?
just a sidenote, i had a college rec where my debate coach said that i often didnt win many competitions, but i tried really hard anyway.. which was a negative since she just had a bad memory and on my app i had dozens of debate awards listed... and someone once said it may not be so bad if negatives are mentioned as long as the positives greatly outweigh, cause then the rec seems more genuine.. it sounded like bull to me, but just a thought. good luck.