The police also stated that all four have been arrested on suspicion of the commission, preparation and instigation of terrorist acts.
"The address and persons connected with it had been under observation by counter terrorism officers as part of an ongoing intelligence led operation."
Police said the probe was unrelated to the arrest earlier on Thursday of a man armed with knives close to the Houses of Parliament.
The 27-year-old was arrested on terrorism charges after being surrounded by firearms officers who pinned him to the ground.
The area around parliament has been on a state of heightened alert since a deadly attack on March 22 perpetrated by Khalid Masood, a 52-year-old convert to Islam.
Masood drove into pedestrians, killing four, and stabbed a police officer to death at the gates of parliament before being shot dead by an armed officer.