Around 128 pupils and staff were evacuated from Darul Uloom School, in Foxbury Avenue, Chislehurst, on Saturday over a suspected revenge attack in reaction to the killing of soldier Drummer Lee Rigby on the streets of London last month.
Two 17-year-olds and two 18-year-olds were being questioned on suspicion of arson, the Metropolitan police said, adding that security and patrols have been stepped up around Islamic centres in London.
"There is an increased police presence around potentially vulnerable locations in the borough of Bromley and across London," a spokesperson said.
A school spokesperson, Sayed Mahmood, said he was "deeply saddened" and that intruders had been caught on the school's security cameras.
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The four arrests came as counter terrorist officers continue to investigate a fire that destroyed an Islamic centre in Muswell Hill last week.
Graffiti linked to the far-right group - the English Defence League (EDL)- was found on the outside of the building.
No one has been arrested in connection with that incident.
Fiyaz Mughal, director of the conflict resolution charity Faith Matters, has said too many mosques remained vulnerable to attack in the aftermath of Woolwich.