Japan hanged four convicted killers Thursday, amid criticism that the country's secretive and slow-moving justice system leaves inmates to languish on death row for decades. Japan, one of the few industrialized countries that still has capital punishment, has boosted the pace of executions in recent years. The country hanged 15 people last year - the most since 1975 when 17 were executed, Justice Ministry official Katsuhisa Nagata said. Though capital punishment has broad support in Japan, which is among the safest nations in the world, many critics say the system needs to be reformed because condemned criminals are often left waiting on death row for years. There are currently about 100 people on death row. Others say the system is too secretive. Inmates do not know when they will be executed until the morning of their hanging. Executions are conducted in secret, and lawyers and family are only notified after the fact.