Well in the case that she's a nurse, I'm sure the places you apply would be very interested in what she has to say because she has probably worked with many other doctors. If it was just a regular patient I would say don't save it, but a veteran ortho nurse would be a good letter considering you want to go into ortho. Especially since she has connections to your ortho program I think this would help you significantly at your home school. At other schools who aren't as familiar with this woman, I think if you preface the letter with the fact that she has worked in orthopedics so she knows what good care is about the comments will have more weight. I wouldn't use her letter if you only have 3 letters, but to use her as your 4th LOR for your ERAS would be pretty good.