I have had 3 Fraxel 1550 treatments spaced 4 weeks apart. Is there any disadvantage in waiting 3 months for my 4th treatment ?

Fraxel laser is a fabulous laser and I generally recommend waiting one month apart between treatments.  There is no disadvantage for you waiting three months for your next treatment.  The skin will still be remodeling new collagen after many weeks.

In general, treatments are about 3-6 weeks apart to get through the series the quickest.  On the longer side for darker skin types.  There is no problem with having a longer interval than that.  If the procedure is done with the Fraxel Dual 1550, usually only 3 are recommended because of improvement made in the handpiece and software.

You will have more collagen stimulation by keeping your Fraxel laser treatments four to six weeks apart, but you will still have continued improvement if you wait three months.