On June 27, 2019, about 1810 eastern daylight time, an experimental, amateur-built RV-4, N102GL, sustained substantial damage when it was involved in an accident near Williamson, Georgia. The commercial pilot received serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he had just taken ownership of the airplane and was departing the previous owner's airport. He performed a preflight inspection, but did not sample the fuel or visually confirm the fuel quantity. The pilot stated that on an earlier flight, the previous owner told him that when one fuel tank was 25% lower than the other tank, he would switch tanks to keep the fuel levels balanced. He started the engine and taxied to the end of the runway. During the taxi, one fuel tank was 25% lower than the other tank, so he reached down and switched the fuel selector to the other tank. He then performed an engine run-up and departed to the east. The airplane lifted off about halfway down the 2,500-ft-long turf runway with the engine producing 2,700 rpm. About 150 ft above ground level, the pilot reduced engine power to 2,500 rpm, but seconds later, the engine lost all power. The pilot performed a forced landing to treetops straight ahead. According to a Federal Aviation Administration inspector, the airplane sustained substantial damage to the engine mounts and the left wing. An inspection of the cockpit revealed that the fuel selector valve was positioned about 2/3 between the right fuel tank and the left fuel tank; closer to the left tank. The carburetor was disassembled; the bowl contained no fuel and about 1 ounce of fuel was found in the accelerator pump cavity. The fuel pump and fuel lines were removed as a unit, with about a teaspoon of fuel present, and the fuel pump was functionally checked and pumped normally. The engine compression and continuity of the accessory drive train was checked, and no anomalies were found. The magnetos produced spark. The oil filter was removed, disassembled, and found to be clean.