Police were called to Clarence Gardens near Regent’s Park at about 10.10pm on Saturday after residents heard the shots.
A large crowd was attending the barbecue in the communal park when up to five men arrived and “indiscriminately discharged a firearm”, police said.
They injured two teenage girls, aged 19 and 17, a 28-year-old man and a 73-year-old woman.
Firearms officers and paramedics attended the scene and the victims are recovering in hospital.
The incident is not being treated as terror-related.
DS Val John-Baptiste, of Specialist Crime North, said: “I need to hear from anyone who witnessed the incident or saw anything suspicious in and around Clarence Gardens.
“We thoroughly investigate every shooting in London – every incident and every weapon on our streets is of great concern to us.
“We know that Londoners feel exactly the same, and that’s why it is vital that anyone with information shares with us what they know, either directly with police or anonymously via Crimestoppers.”
A Section 60 Order was authorised across the Camden borough.
Detectives are appealing for witnesses or anyone with footage from the party. Anyone with information is asked to call police on 101 quoting reference Cad 7619/14Aug.