The incident happened around 4:30 pm near the intersection of 14th Avenue and York-Durham Townline.
One worker was airlifted to hospital with serious but non-life threatening injuries and two others were transported to Markham Stouffville Hospital. The fourth person was not seriously injured.
The cause of the explosion has yet to be determined. The contractor, Strabag, will be conducting its own investigation into the incident, in addition to an ongoing Ministry of Labour investigation.