Hi, welcome to BYC!
Sorry your girl is having trouble.
4 # seems like it would be heavy for a bantam to me. What breed?
I ALMOST have to disagree with your vet about feeding layer... with the exception of asking WHAT were you feeding before... including treats and supplements? IF she was eating scratch as her sole ration... you are very likely seeing the result of long term nutrient deprivation. In which case layer would be better. My preference is flock raiser, grower, or unmedicated starter. Protein and it's amino acids are HIGHLY important. Even though most feeds are mostly corn, the difference is scratch doesn't have the added amino acids, vitamins and mineral that ANY of the formulated rations will contain. Feeding "layer" usually has 4% ish calcium. For birds that aren't laying that is too much and *can* )doesn't mean will) cause gout or even kidney failure if fed long term.
Sounds like it COULD be either a severe vitamin deficit or Marek's. Best resource I have found regarding Marek's is...
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
No.. I don't think you should force feed her. She will die of dehydration before starvation. Get some vitamins and electrolytes into your water. Simple recipe is... 2 cups warm water, 2 tablspoons brown sugar, 1/2 teaspoon each salt and baking soda. Stir until dissolved and serve full strength. If you have poultry nutri drench add some in. Consider getting if you don't. Baby vitamins with NO iron can be used in a pinch.
Did vet even run a fecal float to check for worm or cocci load?
Here is a tool that MIGHT help you narrow down the illness according to symptoms...
http://www.poultrydvm.com/symptoms
I rarely see it work, but all cases are unique... Selenium and vitamin E could help.
Scrambled egg is an excellent source of easy to digest nutrients... and would be what I offer... In addition to other higher protein snack like tuna, mackerel, chicken, cooked beef, live meal worms. If she doesn't have any grit while confined.. that may have some impact.
Adding your general location to your profile might help others make better recommendations. Do you mind sharing how much your vet visit costed? Were there any mention of checking for parasite or what possibilities it might be? Sadly (and thankfully) BYC is often a better resource than many vets.

Glad you found us.
Hope she feels better!
Any help or thought...
@Eggcessive,
@Wyorp Rock,
@casportpony