The warrants were signed by the state despite global appeal from human rights activists.
President Goodluck Jonathan had directed state governors to exercise their constitutional rights by signing death warrants of death row inmates in order to reduce the rising level of criminality in the country.
Those executed include Chima Ejiofor, Daniel Nsofor, Osarenmwinda Aigbonkhan and Richard Igagu.
Confirming the execution, Edo State Attorney General and Commissioner for Justice, Mr Henry Idahagbon said the executed men were convicted 15 years ago, noting that the Edo State Government had nothing to do with the execution.
He said: “These people were convicted 15 years ago. I was only informed this (last) night by the prison authorities that they had been hanged. One of them was convicted in Kaduna, while their matters had gone up to the Supreme Court, and came back to the Federal High Court, Benin. It really has nothing to do with us as a government.
“The governor only signed the death sentence of two, while previous governors signed that of two others. The information I got was that they went to the Federal High Court, which Monday refused their plea for leniency and I believe what the prison authorities did was to execute them immediately after they left the court.
“Edo State government has no hand in it. I was only informed that it had been done,” he added.
In the suit before the Federal High Court, the death row prisoners had contended that to execute them after over 16 years of trauma, suspense and imminent death would amount to cruel, inhuman and degrading treatment.
They had asked the court to order the Edo State Governor to commute their death sentences to terms of imprisonment.
Amnesty International said it has “received credible reports that authorities in the state of Edo in southern Nigerian have hanged four men in Benin City Prison on Monday – the first known executions in the country since 2006”.
Noting that “A fifth man remains at imminent risk of execution”.
Lucy Freeman, deputy director for Africa at the organization, said:
“If confirmed, these executions mark a sudden, brutal return to the use of the death penalty in Nigeria, a truly dark day for human rights in the country.
“We again urge the Nigerian authorities to stop all executions immediately and return to the moratorium on executions in the country. We oppose the death penalty in all cases without exception, as it is the ultimate cruel, inhuman and degrading punishment.”
According to Amnesty International’s Death Sentences and Executions 2012 report, Nigeria sentenced 56 people to death last year, and approximately 1,000 people are reportedly on death row in the country.
Legal Defence and Assistance Project (LEDAP) in a statement by its National Coordinator, Mr. Chino Obiagwu, said: “The Attorney-General of Edo State and the prison authorities were duly served with the court processes, comprising the notice of appeal and motion for stay of execution.
“Under Nigerian laws, an appeal and application for stay of execution should restrain further action until the appeal is determined.
“By executing the prisoners, Nigeria government has demonstrated gross disregard to the rule of law and respect for the judicial process.”