Led By Donkeys covered a street in Kensington in yellow and blue paint ahead of the first anniversary of the war in Ukraine.
The group said it wanted to remind Russia's president of Ukraine's "right to self-determination".
The Met Police said three men and one woman remained in custody.
Led By Donkeys, which began in 2018 as an anti-Brexit group, said it had poured 170 litres of yellow paint on the eastbound carriageway of Bayswater Road and a similar amount of blue paint on the westbound side, with passing traffic then spreading the colours along the road.
Explaining the protest in a tweet, the group said: "Tomorrow is the first anniversary of Putin's imperialist invasion of Ukraine, an independent state and a people with every right to self-determination.
"The existence of a massive Ukrainian flag outside his embassy in London will serve to remind him of that."