A Dorchester three-decker was evacuated and four residents were taken to the hospital to be checked for carbon monoxide exposure Saturday after they complained of lightheadedness, according to fire and public safety officials. Firefighters arrived at 6 Millet St. just after 9:30 a.m. said Boston Fire Department spokesman Steve MacDonald. Emergency Medical Service officials found evidence of elevated carbon monoxide levels in residents’ systems. Four said they felt dizzy, lightheaded, and nauseated, and were taken to Massachusetts General Hospital. Firefighters believe the source of the leak was a faulty stove on the second floor.