The ship, named by the navy as the MV Asso Ventuno, was boarded on Sunday about 40 miles (65 km) off the coast of Nigeria’s oil-producing Bayelsa state, according to the International Maritime Bureau (IMB).
There were no injuries to other crew members and the pirates released the ship, which continued to a safe port, IMB said.
A spokesman for Nigeria’s navy confirmed the incident: “Five vessels have been deployed for patrol duties off Brass Area.
“The Nigerian Navy in collaboration with other agencies have intensified search and rescue efforts of the four kidnapped crew members of MV Asso Ventuno and apprehension of the pirates.”
Piracy and kidnapping in Nigeria’s oil-producing Delta and offshore are common and the oil-rich Gulf of Guinea is second only to the waters off Somalia for the risk of pirate attacks, which drives up costs for oil and shipping firms.
There have been a string of kidnappings in Nigerian waters and in the restive oil-producing Niger Delta this month. Last week, pirates looted an oil tanker off Nigeria and abducted five crew, the ship’s operator said.
Four South Koreans and two Nigerians were snatched in Bayelsa earlier this month but were later released. Nigerian criminal gangs operating in the Niger Delta usually free captives after a ransom is paid.
Gunmen abducted two Lebanese men working for Nigerian construction company Setraco in Delta state this month, and killed a soldier protecting them. They are still missing.
The 83-year-old mother of Finance Minister Ngozi Okonjo-Iweala was kidnapped on December 9 in Delta state but freed five days later after a military search.
In the north of Nigeria, where Islamist militants operate, another form of kidnapping of foreigners has emerged this year.
A Nigerian Islamist group said on Sunday it was behind the kidnap of a French national last week, citing France‘s ban on full-face veils and its support for military action in Mali.
Western governments are increasingly concerned that Islamists in northern Nigeria are linking up with outside groups, including al Qaeda’s north African wing.
(Reporting by Tife Owolabi; Additional reporting and writing by Joe Brock; Editing by Jon Boyle)