It is extremely important to find the root cause of the anorexia and follow the advice of your veterinarian. They have given your dog an exam and taken a history and have a much more complete picture of the condition of your dog which is the first step towards this.  I can only give general advice on causes for inappetence as a dog not eating for 4 days means there that the cause needs to be resolved.

A dog not eating can have many causes such as feeling ill from causes such as pancreatitis, an obstruction, kidney disease, systemic infections, liver problems, dental disease making it painful to eat, behavioral issues, extreme anxiety or a number of other issues.  Here is an article that goes over many of these causes in more detail as well:  https://pets.webmd.com/dogs/guide/dog-not-eating-possible-causes-and-appetite-solutions#1

To find the cause of what is causing it, there are often many tests that help rule out the causes.  Again, the first being a thorough history and physical exam.  Other tests could include blood work, x-rays, or a number of other tests depending on what the veterinarian finds.  Again, it is very important to work with your veterinarian to either confirm the diagnosis or rule out the more serious causes as quickly as possible.

What to do to encourage feeding for your dog’s specific cause generally relies on the symptoms and it is important to ask for advice on food from your veterinarian that has examined the dog as they would have a much better idea.  However, here is an article that talks about some things that can be tried:  https://www.petmd.com/dog/nutrition/6-appetite-stimulants-dogs