The number of Americans claiming new jobless benefits fell back to a four-year low last week and manufacturing in the north east held up in March, providing more signs the US economy was firmly on a self-sustaining growth path. Initial claims for state unemployment benefits dropped 14,000 to a seasonally adjusted 351,000, the Labor Department said yesterday. That took claims back to a four-year low reached in February. The New York Fed’s general business conditions index rose to 20.21 – its highest level since June 2010 – from 19.53 in February. However, a jump in prices paid by manufacturers in New York state and the biggest gain in five months for US producer prices in February hinted at potential headwinds facing the economy.