Thirteen firefighters were sent to tackle the blaze believed to have been started by butane lighter fluid canisters in a store room, which overheated near a fan heater.
Crews were called to the petrol station, in Spring Hill, off Dudley Road, near Winson Green, just after 11am today.
Two teams of firefighters from Handsworth and one crew from Ladywood attended the blaze.
The A457 Dudley Road has been blocked by the incident.
West Midlands Fire Service spokesman Neil Spencer said four people received treatment at the scene. One man suffered burns to three per cent of his face. Another man had 15 per cent burns to his face and hands. A third also had burns to their face and hands.
A woman was treated at the scene for burns to four per cent of her hand.
They were taken to either City Hospital or Queen Elizabeth Hospital for treatment.
The blaze happened in a single storey building measuring around 50ft by 32ft at the petrol station.