You will only get the complete answer from a surgeon who does ACI. Personally, I wouldn't want to go through major surgery (2 operations indeed) and 2 years of heavy rehab (which you'll have to pay for if not insured presumably on top of the surgery costs) unless I was through to the bone, on the thinking that the defect may spread. I do have a grade 4 focal defect that is currently filled with fibrocartilage - it is tender all the time, however while it is still liveable I am not going near more surgery.
If you have a lot of pain, why not try injections first - hyaluronic acid, cortisone, PRP? The pain does not need to be permanent