How long does it generally take for one's body/hormones to regulate after going off a corticosteroid that has been used for the past 4 years?

Not much: so long steroids were tapered right, you might need still stress dosing if going through surgery or if you acquire severe infection for some time after stopping the steroids, more important though are the other effects of steroids on your body, especially eyes and bones, also your blood pressure, and your blood sugar metabolism, you need to keep following with your doctor, and endocrinologist