The casualties remained in hospital last night.
The crash happened at about 7:45am on Kilbowie Road in the West Dunbartonshire town.
Emergency services were called to the scene and four people, including the 52-year-old man driving the van, were taken to the Queen Elizabeth University Hospital in Glasgow.
The three others injured were the teenage boy, a 42-year-old man and a woman aged 48, Police Scotland said.
A white Renault van, with utilities firm Hiflow markings, could be seen behind the police cordon along with the smashed bus shelter as officers investigated.
A visibly shaken man at the scene said: “I saw one woman fly about 30ft through the air.
“Another guy was left mangled against the fence.
“It looked a young boy was left in the mess.
“It took at least ten minutes for any police or ambulance to get here.
“There’s always crashes here at this junction because people speed round the corner.”
John Dunsmore, whose salon is beside the bus stop, said: “I got a call about 8am to say there’d been a crash.
“I got here and there was fire engines and ambulances and loads of police. It was carnage.
“Loads of people belt up and down that road. It really is a terrible road because people speed up and down and there is a blind spot as you come round.”
Chris McLean, who walked past the scene minutes after the crash, said: “It didn’t look good - lots of emergency services, road was closed off. I could see someone on a stretcher receiving treatment.”
All of the casualties remain in the care of medics, officers confirmed.
The road was closed following the crash to enable investigators to carry out their inquiries at the scene.
Inspector Adam McKenzie said: “We are in the process of establishing the exact circumstances of the incident.
“I would like to appeal to people who were in the area at the time to contact us.
“If you saw the van or the incident, please do contact us and pass the information on.”
Anyone with information is urged to call officers at Dumbarton police office via 101, quoting reference number 0557/28.