SHANGHAI, Oct 21 (Reuters) - China’s central bank lifted its official yuan midpoint to a new four-month high on Thursday, breaching the psychologically important 6.4 per dollar level for first time since June. The People’s Bank of China (PBOC) set the midpoint at 6.3890 yuan per dollar prior to the market open, 179 pips, or 0.28%, stronger than the previous fix of 6.4069. It was the firmest since June 11. The official guidance rate largely matched market forecasts, traders said, and it was 6 pips weaker than Reuters’ estimate of 6.3884. (Reporting by Winni Zhou and Andrew Galbraith Editing by Shri Navaratnam)