I have been working with vets for over 25 years, before they even had "vet techs". I can check horses mouthes and check for points and edges, but would never tell someone that "their horses mouth is just fine". I can give shots, draw blood, to exams, take exrays , read lab reports, even suture, but would never presume to tell another person that their animal is okay. Just because she took all the classes doesn't mean she is a vet or even a vet tech.
Whenever a horse doesn't put on weight the first thing I think of is either not getting enough food by WEIGHT, not scoops or flakes, or teeth issues. A four year old could have cups on their teeth or any number of things that is making her hurt to eat very much. Ulcers as suggested is also something to rule out.
I would suggest you leave the diagnosis to your vet and not your barn owner and have your vet come out and take blood, check her teeth and give her a good checkup. Don't wait until she is down so far on weight or health that you have a hard time bringing her back.