The men were thought to have been arrested together in a street in north- west London by officers working for a specialist squad set up to target gun crime under the banner Operation Trident, Scotland Yard said.
The four were being questioned about the recent wave of shootings in London. There have been 13 killings and at least 30 other shootings in the capital this year in Yardie-style turf wars believed to be connected to the lucrative crack cocaine trade.
Earlier this month, a small-time drug dealer, Dean Roberts, 20, was shot several times at close range in Harlesden, in north-west London.
And in May, Laverne Forbes, 28, and her partner Patrick Smith were murdered in front of their seven-year-old daughter in north London.
The shootings have led to a leap in London's murder rate, and police have put more armed officers on the streets as part of Operation Trident.Reuse content