The pilot took off on a VFR flight plan with a litter patient and 3 other passengers onboard. The right main fuel tank was used for about the first 30 minutes of flight, then the pilot switched to the left main tank. While en route, an IFR flight plan was obtained when the destination weather did not improve to VFR conditions as forecast. The pilot reported that before descending into the clouds for an ILS approach, he performed a before landing check and switched back to the right main fuel tank. The engine lost power abruptly while on the approach. The pilot believed that he switched to the right tip tank, and then later, back to the right main tank, while attempting to start the engine. As the aircraft broke out of the clouds, the pilot attempted to maneuver to a clearing, but the plane struck trees and crashed. The fuel selector was found positioned to the left tank which had about 3 pints of fuel remaining.