Severe restrictions on swimming and water sports have been extended in the Egyptian resort of Sharm el-Sheikh, following the first report of a team of experts on a recent series of attacks.
The BBC reports at least two sharks are now believed to have involved in the attacks.
Marine biologist George Burgess said a single shark carried out two of the attacks, while a different species was responsible for two others.
He told the BBC that environmental effects had caused the "highly unusual" spate of attacks and said a shark hunt was useless.
"What you have here are rational attempts by a predator to find food," Mr Burgess, said.
He advised people to swim in groups in areas inside the reef, and to stay out of the water at night.
An elderly German woman was fatally mauled just offshore on Sunday, a day after beaches were reopened following shark attacks that injured two Russian snorkellers on 30 November and another one on 1 December.
The BBC reports that many of the main beaches have been closed to swimmers and snorkellers since Sunday's fatal attack.
South Sinai governor Muhammad Shousha said on Wednesday that one of the sharks involved in the attacks - a mako - had been caught. But the other shark was still at large.