The incident is believed to have occurred late on Saturday night, with emergency services called on Sunday morning when another sailor noticed the upturned boat on the shores of a lake in Örnsköldsvik, north eastern Sweden.
Two bodies were found under the boat, with a further two beside it. Three of the passengers were pronounced dead on the scene, with the fourth critically injured and rushed to hospital. The deceased passengers were two women, aged 27 and 28, and a 43-year-old man.
An emergency search party was launched with police unsure if the boat held more than just the four passengers at the time of the crash.
Police suspect that the vessel may have collided with a buoy 500 metres from where the boat was found, a theory shared by locals in the area according to the Aftonbladet newspaper, although the claims remain unproven as yet. The vessel sustained serious damages to the rudder.
The man who survived the incident was the driver of the boat and was still in hospital on Monday.
Police have launched a forensic investigation.