Will reversing a genioplasty 4mm backward cause skin sagging ? Is it too early to get a reversal done? (photos)

So I had a 6mm forward sliding genioplasty 2.5 months ago. The swelling has gone down a lot but I still think about getting it reversed. I already had a pretty long face and the surgery makes my face even longer and fatter. I know that I won't gain my exact old looks back but I think that a subtotal reversal (like 3-4max mm) would suit me better. Is 4mm forward too much? I am 18 years old and I am afraid my skin will sag. Also I could get it done in February (5months postop). Is it risky?