China’s yuan hit a four-year low on Wednesday, falling for a second day after authorities devalued it in a move that sparked fears of a global currency war and accusations that Beijing was giving an unfair advantage to its struggling exporters. Spot yuan fell to 6.43 per dollar, its weakest since August 2011, after the central bank set its daily midpoint reference at 6.3306, even weaker than Tuesday’s devaluation.