Pain persisting 4 weeks after Herpes zoster is called Post-herpetic neuralgia which occurs because of damage to nerves. It requires aggressive treatment in the form of drugs, nerve blocks, local creams and sometimes even injection in the spine depending on the site of pain. This pain should not be allowed to linger on as the treatment options are very few if it is more than 1 year old pain. You should consult a Pain Clinic in a hospital or a neurologist for initiating medications. The Pain Clinician will administer nerve blocks for relief of pain.