I had 4 wisdom teeth removed 4 days ago & I still have severe numbness on the left half of my lower lip & chin, should I be concerned?

Mental dental. Sounds like trauma to the left mental nerve (from the Latin for chin, not brain) presumably from the needle used for local anesthesia. Call your dentist; he/she will want to know. Ask him/her how long they think it will take to get better & do they want to see you again. I'm not a dentist but I assume it will eventually recover; but don't be meek about getting SOME kind of answer.
Call your surgeon. They need to know of this post operative condition. Medications and therapy may be indicated to help healing and regaining sensation on your left side.