Mr. Couch’s parents, Fred and Tonya Couch, own homes in Fort Worth and the nearby suburb of Burleson, where the crash occurred. Fred Couch runs a sheet-metal company. On the night of June 15, Ethan Couch and several friends stole beer from a Walmart and went to his parents’ Burleson home to have a party. Later, he and seven others crowded into the pickup truck owned by his father’s company to go to a store.

Prosecutors said Mr. Couch swerved off Burleson-Retta Road as he was driving about 70 m.p.h., striking and killing four pedestrians: Breanna Mitchell; Hollie Boyles and her daughter Shelby, 21; and Brian Jennings. Tests showed that Mr. Couch had a blood-alcohol concentration of 0.24, three times the legal limit for drivers, the authorities said.

Hollie and Shelby Boyles had left their house that night to assist Ms. Mitchell, a stranger whose car had broken down. “I’m sure the judge is doing what she thinks is probably right for Ethan’s rehabilitation,” said Eric Boyles, Shelby’s father and Hollie’s husband. “but from the victims’ standpoint, she underestimated the impact. Words can’t describe how disappointed I am in terms of how the judicial system works.”

Two teenagers riding in the bed of the pickup were thrown out of the vehicle and severely injured. One of them, Sergio Molina, 15, suffered a brain injury and remains in a minimally responsive state. His family filed a negligence suit against Mr. Couch, his parents and his father’s company. The suit accuses Mr. Couch of having a prior alcohol-related arrest in February at the age of 15.

“We are disappointed by the punishment assessed, but have no power under the law to change or overturn it,” one of the prosecutors, Richard Alpert, said in a statement. “Our thoughts and prayers are with the families and we regret that this outcome has added to the pain and suffering they have endured.”