US President Barack Obama hailed the full implementation of an international nuclear accord with Tehran and the release of Americans held prisoner in Iran as a vindication of "strong American diplomacy".
"Today is a good day," Obama said in a White House speech on Sunday. "When Americans are freed and returned to their families, that's something we can all celebrate."
A charter plane with the Americans has left Tehran for Switzerland, Xinhua reported.
The release of the four Americans, including a Washington Post reporter Jason Rezaian, was part of a prisoner swap between Iran and the US.
US media said Washington would either pardon or drop charges against seven Iranians in exchange for the Americans.
"Engaging directly with the Iranian government to sustain basis for the first time in decades has created a unique opportunity, a window, to try to resolve important issues," Obama said.
The president's speech came at a time when his government was lifting billions of dollars in international sanctions on Iran as part of the international nuclear accord.
The P5+1 group, namely the US, Britain, France, China, Russia and Germany reached the comprehensive nuclear deal with Iran on July 14, 2015.
The accord, known as Joint Comprehensive Plan of Action, would provide sanctions relief for Iran in exchange for limits on its controversial nuclear programme.
Talking to the people of Iran, Obama said, "For decades your government's threats and actions to destabilise the region has isolated Iran from much of the world. Now our governments are talking to each other."
On Saturday, Obama signed an executive order to lift sanctions on Iran after international inspectors concluded that Iran has dismantled large parts of its nuclear programme as promised.
However, hours after the release of Americans, the Obama administration also announced new sanctions on Sunday on individuals and 11 entities involved in Tehran's ballistic missile programme.
"We will continue to enforce these sanctions vigorously," Obama said. "We are going to remain vigilant about it."
Also on Sunday, the US announced that Iran will receive $1.7 billion in debt and interest as Obama pledged that it was time to solve the financial disputes between the two countries.
Iran will receive $400 million trust fund and roughly $1.3 billion compromise on the interest.
In 1981, the Iran-US Claims tribunal was established in The Hague to settle debts between the two countries.
Obama said the settlement between the US and Iran at The Hague would save US money. There was no point in dragging out that dispute, he said.
