At around 6.40 pm (12:10 am IST) all four men were arrested on suspicion of criminal damage and being unlawfully on diplomatic premises.
It is understood the protesters did not manage to get inside the building.
Iranian foreign ministry spokesman Bahram Ghasemi said Tehran made a strong protest to the British government over the incident, according to the IRNA state news agency.
The report said supporters of an "extreme cult" living in Britain were responsible.
Iran's deputy foreign minister Abbas Araghchi made his protest known to the British ambassador in Tehran, demanding "absolute protection of our diplomats in London and immediate dealing of the police with the attackers", IRNA said.
In 1980, six gunmen who supported a dissident Iranian group took over the Iranian embassy in London, taking more than two dozen hostages.
After a six-day siege, British special forces raided the building, killing five of the gunmen and arrested another. Two of the hostages also died.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)