Merced County Sheriff Vern Warnke, who also serves as coroner, gave the new total after canvassing hospitals and checking with coroner's officials in a neighboring county. He said he knew of no deaths among those taken to hospitals.
Shortly after Tuesday's crash, a California Highway Patrol officer at the scene said five people died.
The dead include a married couple from Mexico who were traveling to visit a daughter in Pasco, Washington, and a man from Avalon, California, whose spouse was hospitalized, Warnke said.
A fourth victim was only identified as a man in his late 30s whose family was being sought through a Mexican consulate.
Investigators, meanwhile, were working to find out why the bus suddenly veered sharply off the highway, smashing into a pole that tore through the vehicle.
Eighteen people were injured, but none of the victims remained in critical condition. Warnke has said rescuers pulled "bags of body parts" from the bus along with survivors, some of whom suffered severed limbs.
Moments before the crash, it appeared the driver was trying to pass another vehicle that he apparently expected to yield, but it would not let him merge, said a passenger headed from Mexico to her home in Pasco.
"It kind of ran him off the road," Nakia Coleman told Washington state's Tri-City Herald newspaper.
Investigators have not been able to interview the 57-year-old driver, Mario David Vasquez of the Los Angeles area, because of the extent of his injuries, CHP Officer Moises Onsurez said.
The bus was traveling north on State Route 99, through the center of California farm country, and was only a couple of miles from its next stop when the crash occurred. It was approaching Livingston, a farming and industrial town of about 13,000, where it was scheduled to change drivers before continuing on to the Northwest.
Passenger Leonardo Sanchez was sleeping peacefully before dawn on the bus carrying him to Oregon to pick blueberries when he was suddenly thrown face-first into the back of the seat in front of him.
"There was lots of screaming and crying," Sanchez told The Associated Press hours after the crash.
He said only about eight people, including himself, escaped largely unscathed. The accident sliced the bus from front to back, with the vehicle coming to a stop when its first rear axle hit the pole.
"We couldn't pull people out because there was shattered glass everywhere, seats destroyed," Sanchez said.
Some victims were thrown from the bus and landed in a ditch. In all, 18 people were admitted to four hospitals, three initially in critical condition. They have been upgraded to serious condition, and the others have lesser injuries, officials said.
Autobuses Coordinados USA was operating the bus. Calls to various telephone listings for the company have gone unanswered.
Its counter at a Los Angeles depot was open Tuesday, but nobody was present when an Associated Press reporter arrived midmorning. A sign in Spanish advertised daily 7:30 p.m. departures to destinations up the West Coast to Washington.
The Federal Motor Carrier Safety Administration listed the carrier as having a "satisfactory" rating as of May 17. The bus was inspected in April and had three violations, including a lack of or a defective brake warning device.
Some highway signs, like those for the speed limit, have support poles designed with points that break away during a crash. But the poles supporting the much larger overhead signs like the one the bus hit are designed to "stay put," said Vanessa Wiseman, a spokeswoman for the state Department of Transportation.
Weber reported from Los Angeles. Associated Press writers Alex Veiga, John Rogers, John Antczak and Amanda Lee Myers in Los Angeles contributed to this report.