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

With genioplasty, a change of 1-2 mm is not going to provide a visual clinical change. It is normal to appear looking longer after the procedure because recessive chins appear short. It is all about facial balance. From the front, I think you have a nice looking chin. A full frontal photo and a profile photo would be extremely helpful to provide better advice. Reversing the procedure is possible but it requires going through the same surgery again (cutting the bone). If you like the overall result but want small changes, you might want to consider shaving certain aspects of the chin.