SC4 has extended walk-in registration hours for winter semester  St. Clair County Community College’s winter semester classes begin the week of Jan. 9.  Walk-in registration hours at SC4’s One-stop Student Services Center on Stone Street at Glenwood Avenue in Port Huron will be extended beginning Jan. 3 to accommodate registration for winter semester classes.  The hours will be:   Jan. 3 to 6 — 8 a.m. to 6 p.m. Tuesday through Thursday and 8 a.m. to 4:30 p.m. Friday.   Saturday, Jan. 7 — 9 a.m. to 1 p.m.  Extended hours for dropping/adding classes and late registration will be:   Jan. 10 to 17 — 8 a.m. to 6 p.m. Monday through Thursday and 8 a.m. to 4:30 p.m. Friday.