The rights group says the men were hanged following unfair trials. The four were found guilty of taking part in a series of deadly bombings in the oil city of Ahvaz in 2005.
They were among a group of 10 ethnic Arabs convicted of acting against the country's security. Three of those convicted were executed last month.
Rights groups and the UN had called on Iran to halt the executions.
Amnesty International says executions in Iran continue at an alarming rate. The group recorded at least 177 executions in 2006 but it fears that the true figure may have been much higher.