SHANGHAI, Aug 20 (Reuters) - China’s central bank on Friday set its official yuan midpoint at the weakest level in four months to reflect falls in spot prices a day earlier. The People’s Bank of China (PBOC) set the midpoint at 6.4984 yuan per dollar prior to the market open, 131 pips or 0.2% softer than the previous fix of 6.4853, and the weakest since April 21. However, the weakened official guidance rate has pushed China’s trade-weighted yuan basket index higher again to 99, the highest since March 9, 2016, according to Reuters’ calculations based on official data. Reporting by Winni Zhou and Andrew Galbraith; Editing by Jacqueline Wong Our Standards: The Thomson Reuters Trust Principles.