The yen has never fallen for four straight years against the dollar, and Japan's former top currency official sees better than 50 percent odds that its currency will halt the losing streak.

While the weak-yen trend from 2012 is still intact, the gap between actual currency levels and purchasing power parity has stopped widening, said Toyoo Gyohten, who was vice minister for international affairs at the Finance Ministry from 1986 to 1989.

The yen fell 12 percent in 2014 to log its a third annual loss, the longest streak since 1997.