The crash occurred Thursday afternoon at Princeville Airport, shortly after a Heli-USA Airways helicopter pilot radioed that he was having hydraulic problems, according to a statement from Kauai Fire Chief Robert Westerman.
Nigel Turner, CEO of Las Vegas-based Heli-USA, said the aircraft was at the end of its tour and minutes from its scheduled landing when it crashed.
"We are in the process of notifying the families of those individuals involved and our sincere condolences goes out at this time," he said. "We are working with authorities to find out exactly what happened."
Of the two men and two women who died, three were killed in the crash and one died while being taken to a hospital, Kauai County spokeswoman Mary Daubert said.
The pilot, identified as Joe Sulak by the Honolulu Advertiser, had more than 10,000 hours flying an A-Star, the type of helicopter that crashed, Federal Aviation Administration spokesman Ian Gregor said.
Sulak was known as "Helicopter Joe" and was Heli-USA's lead pilot, a member with a rival company told the newspaper. "I had ultimate respect for the man," Mike Stewart said. "He was a great guy."
The crash comes one month after the Federal Aviation Administration announced new safety standards for air tour companies that operate at many vacation spots nationwide and for pilots who offer rides at air shows. The safety rule does not take effect until August.
The FAA promised to closely monitor deaths and other accidents involving air tours after looking into 107 accidents that killed 98 people from 1988 to 1995.
Thursday's crash was the fifth fatal helicopter incident on Kauai in the past four years, the newspaper reported.
Five people were killed in 2004 when a helicopter operated by Bali Hai Helicopter Tours Inc. crashed into a mountain on Kauai. And three passengers drowned in 2005 after a Heli-USA helicopter plunged into the Pacific Ocean off Kauai.
Turner defended the safety of his helicopters. The company has a fleet of six in Hawaii, and also flies in Nevada.
"The company has flown over a million passengers. This is our second accident in a million people," he said, adding that he would not hesitate to put his own family in his helicopters.