That truly is your best bet.
I'm sorry that you have to go for a 4th time. And I think its great that your baby is doing so well through it all and even still eating. That is very encouraging. But its time to knock this out once and for all.
Its not necessarily the fanciest or most expensive vets that are the best. But you really do need one that knows and understands reptiles, esp. snakes. Treatments and meds work a bit differently in a cold blooded animals.
Please keep us up to date and I wish you and your baby best of luck getting rid of this problem!