This is the 4th week of Precision TX and I have wrinkles under chin that I never had. Why is this happening? (Photo)

Hello.  This is a normal finding after Precision, and can be treated with gentle massage of the area.  Also, be certain that your compression garment is not ill-fitting.  It should rest smoothly across the neck in that area.

These wrinkles are caused by firmness (called "induration") in the subcutaneous fat creating ridges in the skin. This firmness is caused by the natural infiltration of white blood cells and collagen producing cells that are trying to heal your laser treatment. 
It is helpful to apply warm compresses, like a warm wash cloth, and gently massage while looking up, to increase circulation, so this layer can be remodeled. (NEVER use anything hot, only warm, and absolutely never anything warmed in a microwave). Do the warm compresses and massage several times a day. You don't want the collagen producing cells to tether the ridges down to the platysma (neck muscle).
One can also use a Neck Cream (we use Sente Neck Cream) for the massage after warming. This neck cream has a dermal repair growth factor.