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.
Beyonce flaunts bikini body, Blue Ivy in vacation pics
Couple mistakenly served bag of cash at McDonald's drive-thru