Richard Schiefer, senior officer at Salzburg police headquarters, said: "I can confirm they are British but ... we do not know their names at the moment. They ... wanted to get on to a landing position but they did not manage it. They went further down the Salzach and went over a weir."
Mr Schiefer said a second boat carrying nine people on the expedition landed safely before the weir.
Three men and a woman died in the accident and three people were injured. Their identities and the name of the company running the expedition have not been released.
An officer at Zell-am-See police station, near the scene of the incident, said the raft was run by a British-owned adventure holiday company based in Taxenbach. "There are very few boats or rafts which use the Salzach. It is a very fast-flowing, wild river, with rapids. It can be difficult to swim in."
Rivers in alpine countries such as Austria often become flooded and can be transformed into torrents at this time of year.
Michael Dukes, a forecaster at the PA WeatherCentre, said that waterways can easily become swollen due to torrential downpours coupled with melting snow coming down from the mountains. "The weather in Austria has been fairly typical for this time of year, with heavy localised thunderstorms in the past few days.
"The snow which fell during the winter will now be melting and flooding into the mountain streams, causing many rivers to run higher than normal - having an obvious knock-on effect downriver. This is a danger in Austria that the people there will be aware of at this time of the year."
A Foreign Office spokesman confirmed that four Britons, died in the rafting accident.
"We understand that next of kin have been informed and were on holiday in the area," he said. "Our consular staff in Austria have been in contact with both the authorities and the organisers of the rafting trip."
He declined to give any further information about the victims or the name of the company which organised the event.