Officials would not say whether a ransom was paid for their release, China's official Xinhua News Agency reported.
Emmanuel Chindah, the state commissioner for agriculture, said they were freed at about 2 a.m. and appeared to be healthy.
The four were on their way to work when they were abducted Aug. 9.
Kidnappings have become common in the Niger Delta, the country's oil-producing area. More than 500 have been reported in the past seven years, targeting both Nigerians and foreigners, with most of the victims released unharmed after the payment of ransom.