Police released images of the suspects hours after plain-clothes officers shot and killed a man in front of shocked passengers on an underground train following a chase.
Armed officers later swooped on an apartment block in south London, arresting a man in connection with Thursday's failed attacks while rooftop snipers covered them.
It was one of a series of raids across London by hundreds of police, some armed with assault rifles and machine guns.
In the closed-circuit TV images of the suspects, the first man appears in a "New York" sweatshirt and a second man is shown in a white cap and a T-shirt with palm trees.
Two others are in dark clothes, slightly obscured by a poor camera angle.
The photographs splashed on the front pages of newspapers on Saturday put faces to the suspects as police appealed for help in finding them.
It was unclear whether either the man police killed or the man they arrested was among the four suspects, who allegedly fled three underground stations and a double-decker bus on Thursday.
Police also investigated an apparent attempt to set fire to the home of Jermaine Lindsay, one of the suspected bombers.
British police released the images
of the suspects on Friday
The shooting of a man by police took place about 10am. Witnesses said he appeared to be a South Asian, wearing a padded coat in the Stockwell train station in south London.
Police chased him into a carriage, pinned him to the ground and shot him in the head and torso, five times at point blank range, a witness said.
Metropolitan Police Commissioner Ian Blair said the shooting was "directly linked" to the investigation. "The man who was shot was under police observation because he had emerged from a house that was itself under observation because it was linked to the investigation of yesterday's incidents," police said in a statement.
"He was then followed by surveillance officers to the station. His clothing and his behaviour at the station added to their suspicions." Stockwell is just one stop away from the Oval underground station, site of one of Thursday's four failed bombings, exactly two weeks after the 7 July bombings killed 56 people.
Witness Anthony Larkin told the British Broadcasting Corporation (BBC): "I've seen these police officers shouting, 'Get down, get down!"
Another passenger on the train, Mark Whitby, said the man did not appear to be carrying anything, but his coat looked padded.
"They pushed him on to the floor and unloaded five shots into him," Whitby told the BBC. "He looked like a cornered fox. He looked petrified."
Shoot to kill
Heavily armed officers patrolled the British capital with clear instructions to stop human bombers, if necessary, with a shot to the head.
"If you are dealing with someone who might be a bomber, if they remain conscious, they could trigger plastic explosives or whatever device is on them," said Mayor Ken Livingstone. "Therefore, overwhelmingly in these circumstances, it is going to be a shoot-to-kill policy."
"I have just had one phone call saying 'What if I was carrying a rucksack?'"
Muslim Council of Britain spokesman
Muslim lobby groups said they were shocked by Friday's killing and urged a full inquiry.
One group, the Islamic Human Rights Commission (IHRC), said
it was deeply concerned at the apparent "shoot-to-kill" policy.
"IHRC is afraid that today's killing may only be the first
in a series of police killings in the post-7/7 era," IHRC
Chairman Massoud Shadjareh said in a statement.
And as Muslims gathered for afternoon prayers, they voiced concerns about backlash. One mosque was evacuated after a bomb threat.
Inayat Bunglawala, spokesman for the Muslim Council of Britain, said he had spoken to nervous Muslims since Friday's shooting.
"I have just had one phone call saying 'What if I was carrying a rucksack?'" he said.
More than 100 alleged revenge attacks have been reported since the 7 July bombings.