Had chin lipo (laser) 4.5 months ago. Is this band in the center of my neck loose skin, scar tissue or platysmal muscle? (Photo)

In order to determine if a vertical band in the neck after chin lipo is either 1) excess of skin, 2) platysma band, and/or 3) post surgical fibrosis, the patient should look at the mirror and activate the neck muscle (platysma) by smiling and grinding the teeth ( aka making a sardonic smile). If the band gets works in deed it is a platysma band. If the band disappears, very likely it is loose skin. If it is unchanged, but feels hard to touch with the hand, very likely it is a fibrotic band.

Thank you for the question. I would return to your plastic surgeon for re-evaluation. It could be some scar tissue or even residual fat after laser lipo. Sometimes touch up treatment is needed to refine previous work. I would suggest discussing treatment of your platysmal bands while you’re there. Good luck. 

it could be scar tissue. not sure without actually feeling the skin. best is to follow up with your doctor and see what they say. they are the best ones to advise as to how to fix this problem you're having

A plastic surgeon can perform a physical examination to determine what is causing the band.  It could be platysma muscle, fat, scarring, loose skin or other etiologies.