A fire in Sutton killed two sets of twins aged three and four.
Locally, they are Kyson, Bryson, Leyton, and Logan.
The youngsters, who were related, were brought to hospital but could not be resuscitate.
A 27-year-old woman was arrested.
Her identify and relationship to the dead have not been revealed.
Approximately 60 firemen were dispatched to the scene shortly before 7 p.m.
The cause of the fire is unknown at this time, and an investigation is underway.
The youngsters are twins, according to London Fire Brigade deputy commissioner Richard Mills.
‘I can't confirm what is going on with anyone else in the building,' he continued.
Mr Mills praised emergency responders who ‘worked incredibly hard to save these youngsters' and ‘did everything possible'.
Counsellors are helping the crews who responded to the tragedy, some of whom went to hospitals to continue CPR.
‘The sympathies of everyone in the South Area Command Unit are with the families and friends of these four young children who have lost their lives tonight,' said Superintendent Rob Shepherd.
‘I know the local community, Sutton and London will agree.
‘We will remain on scene with our emergency service colleagues through the night.
‘The cause of the fire is unknown at this time and will be examined.'