Two men, a woman and a teenage boy suffered burns to the face, hands, neck and arms in the incident in Severn Side, Stourport-on-Severn at 07:30 BST.
The 17-year-old had significant burns to his hands and face and was airlifted to hospital for specialised treatment.
A man in his 60s, a woman in her 60s and another man in his 20s were taken to Worcester Royal Hospital.
West Midlands Ambulance Service said paramedics were called to reports of an explosion near to the Angel Inn.
The man in his 60s sustained superficial flash burns to his face as well as a neck injury.
The woman had partial burns to her hands whilst the man in his 20s had sustained superficial burns to his arms and neck, the ambulance service said.
Four people have been injured in a suspected gas explosion onboard a boat in Worcestershire.