Nigeria is the fifth largest supplier of oil to the Untied States, and security experts fear new attacks, in combination with parallel attacks in the Mideast, could lead to oil at $120 a barrel and U.S. gasoline prices of $5 to $6 a gallon.
"Every little spike in Nigeria causes us serious problems when it comes to global oil prices," said Ann Korin of the Institute for Analysis of Global Security.
THE BLOTTER RECOMMENDS
In a series of e-mail exchanges with ABCNews.com, the self-proclaimed leader of a group called MEND, or Movement for the Emancipation of the Niger Delta, said previous attacks and kidnappings that had avoided loss of life do not seem to be working.
"We have the capacity to be as ruthless and as callous as attacks witnessed in Iraq," said the writer who goes by the name of Jomo.
In previous e-mail messages, Jomo had correctly predicted a series of car bombs in the oil-producing region.
He said the four, who worked for the AGIP company, were well but "are presently being watched by guards under instruction to shoot them if any attempt is made to release them without proper authorization."
Jomo said his group wants all oil companies to leave the Niger Delta now, blaming them and the Nigerian government for the impoverished conditions of the people who live in the region.
"How can you explain a situation where we account for all nigerias [sic] wealth and we live without electricity in shacks made of cardboard and straw? How can you explain my people drinking from salty creeks in which they bath and defecate?" he wrote.
MEND and other militant groups in the area have launched an increasingly violent series of attacks against foreign oil facilities and workers.
Nigerian officials acknowledge that hundreds of billions of dollars in oil revenues have been stolen or wasted by corrupt officials.
In an interview to be aired tonight on ABC World News and Nightline, the head of the Nigerian anti-corruption agency, Nuhu Ribadu, says the violence "is a direct result of the corruption thing we are talking about."
Ribadu said one of the governors in the Niger Delta has stolen at least $300 million, and "we found he has properties in about eight countries in the world now," Ribadu said.
The governor, who has denied the charges, is in custody, awaiting trial.
In response to ABC News' request for comment, AGIP released the following, "ENI [AGIP's parent company] is working with the Nigerian authorities and the [Italian Foreign Ministry's] crisis unit for a positive resolution to the deplorable affair of the kidnapping of its employees in Nigeria."
AGIP also said, "ENI has had no direct contact with anyone except the Italian foreign ministry and the Nigerian authorities."