Accused Gilgo Beach serial killer Rex Heuermann has been charged with a fourth murder — the killing of Maureen Brainard-Barnes, who was last seen alive on July 9, 2007. Heuermann, 60, was hit with the new charges at his appearance Tuesday at the Suffolk County Courthouse in Riverhead, LI. The Manhattan architect was already accused of killing three young sex workers on Long Island — Melissa Barthelemy, Megan Waterman and Amber Lynn Costello — and on Tuesday, Suffolk County District Attorney Raymond Tierney charged him with the fourth of the “Gilgo Four” victims. Heuermann allegedly murdered th…