The brothers, aged 21, 19 and nine, died at the scene in Clackline in the early hours of Friday morning, along with a 45-year-old man, who was visiting from NSW.

Police Superintendent Gene Pears said the accident site about an hour northwest of Perth was a confronting scene for first responders.

"This is a heartbreaking incident ... unimaginable," he told reporters.

The 21-year-old was driving and the family friend was travelling in the passenger seat when the silver Nissan Navara left the road and hit a tree about 12.30am.

The younger man's brothers were in the ute's back seat.

"When you lose three young men in the prime of their life and their friend, this will have lifelong impacts for that family, for their friends and for our community," Mr Pears said.

"This is a heartbreaking incident for WA ... my heart goes out to the families."

Major Crash investigators are investigating the incident.

Anyone with information or dashcam footage of the ute before the crash is urged to contact Crime Stoppers.

Australian Associated Press