The accident occurred at 10:46 a.m. Wednesday near Portageville on Interstate 55, according to the Highway Patrol in Poplar Bluff, Mo.
The southbound 2001 Chevrolet Suburban carrying the four victims crossed the I-55 median and went into the path of the tractor-trailer in the oncoming lane, said Highway Patrol telecommunications officer Stephen Stillman.
The victims included the driver, 52-year-old Steven Branch, and three passengers -- 74-year-old Robert Thompson, 71-year-old Joseph Downs, and 44-year-old Betty Wimpie, all of Milwaukee.
Dennis Wimpie of Milwaukee, the older brother of Wimpie, said the victims were friends who were going to visit other friends on the holiday.
He also said his sister Betty was a happy-go-lucky person who will be missed. She is survived by four sisters, three brothers and a daughter.
Debbie Foster, telecommunicator for the Missouri Highway Patrol, said none of the four victims was wearing a seat belt. All four were ejected and died at the scene, Stillman said.
The truck driver was 53-year-old John Sparks of Sikeston, Mo. He was in serious condition Thursday in a Missouri hospital, Foster said.
Stillman had no word on the cause of the crash.
WDJT-TV in Milwaukee reported that the vehicle may have blown a tire just before the accident.