Police said the injured were motorcyclists, who were caught in the traffic below or were parking their motorbikes near the transformer.
The explosion happened near the Vanit Building on Wireless Road.
One of the injured was identified as Wichai Piromkit, a motorcycle taxi driver, who had parked his motorbike near the transformer. Fiery pieces of the transformer hit his face and body, police said.
They identified the other three injured people only by their first names – Chaiyo, Thawatchai, and Chayakhom.
All four were rushed to a nearby hospital.
Police said the explosion also damaged four motorcycles.