Police said those arrested were aged 18, 24, 25 and 27.
Four men have been arrested in London on suspicion of planning an Islamist terror attack, weeks after police thwarted two separate alleged plots.
Scotland Yard said the men were detained under the Terrorism Act in east London on Wednesday.
Britain's domestic intelligence agency MI5 was thought to be part of the investigation which led to the arrests.
In March, a man killed five people after driving into a crowd of pedestrians on Westminster Bridge in central London before stabbing a police officer in the grounds of parliament.
A day before, police arrested an individual carrying knives near Downing Street - the London residence of the prime minister.
The police force says the arrests are linked to activity in the United Kingdom, but has not revealed details of the alleged plot.
The arrests were part of an ongoing investigation by the Metropolitan Police's Counter Terrorism Command and MI5 and were linked to UK-based activity.
Security has been intensified in the capital since alleged Isis supporter Khalid Masood killed five people in a vehicle and knife attack in Westminster in March.