A dozen anti-riot police wearing helmets and knee-high protective gear and carrying shields and batons formed a phalanx outside the offices of the Central Provident Fund (CPF) as a commanding officer approached the demonstrators.
"You are committing an offence of public nuisance. If you don't disperse you will be arrested," the officer told the protesters as business people and employees watched in the central business district on Thursday.
The four protesters, among them an office administrator and the sister of an opposition leader, Chee Soon Juan, voluntarily dispersed.
They denied being part of any political group.
Police later asked them to hand over their protest materials - T-shirts and placards - as part of an investigation to determine whether they had violated any laws.
Five-man protests illegal
The four said they did not need a
permit to protest
The two men and two women assembled at lunchtime outside the CPF building in the business district.
They said they did not need a permit and staged their protest for about an hour.
Chee Soon Juan, the leader of the opposition Singapore Democratic Party, stood a few paces away from the protest of four, selling and autographing copies of his book called The Power of Courage.
Under the law, any public protest of at least five people without a police permit is deemed illegal.
The protest took place as Singapore was in the midst of official celebrations of its 40th anniversary of independence.
The protesters hoisted placards calling for greater openness in how the government handles public funds used for retirement pensions, overseas investments and the building of subsidised high-rise apartments.
A police officer said they had received a telephone call from a "member of the public" about the protest, prompting them to send in the riot police.
Monica Kumar, 45, one of the protesters, said they had been insipired by public outrage that followed revelations last month that the chief executive of Singapore's biggest charity, the National Kidney Foundation (NKF), was being paid an annual salary of $350,000.
"We call on the government to make Singapore more transparent and accountable, starting with the state organisations mentioned above"
The scandal sparked an online petition signed by more than 40,000 people and forced the NKF chief executive, TT Durai, and the entire board to resign.
Lack of accountability
"In reality, the NKF is reflective of the entire system in Singapore where public matters are run in a non-transparent and non-accountable manner," the protesters said in a statement.
The statement called on the Housing Development Board and the Government of Singapore Investment Corporation (GIC) to open their books for public scrutiny.
GIC manages more than $100 billion in funds and invests globally.
"We call on the government to make Singapore more transparent and accountable, starting with the state organisations mentioned above," the protesters said.