The brother of a young man missing for four days says his disappearance is “completely out of character”.

Authorities are throwing everything into the search for 21-year-old Finn Coyle, who was last seen about 2.30pm on Friday at his home on Tanner Rd in Carmel, in the Perth hills.

“It’s entirely unlike him (to disappear). It’s completely out of character and we’re seriously worried for his wellbeing,” Coyle’s brother Conor said during an appeal for information.

Know the news with the 7NEWS app: Download today

He said his brother had been battling recent mental health challenges.

Coyle, who was described as “resourceful”, loves spending time in the bush near his home, and that is where police, SES and the Department of Fire and Emergency Services are concentrating their efforts.

The search began on the weekend but was ramped up on Tuesday due to wintry conditions and the growing time since his last contact.

Police believe he is not too far from home and is on foot.

“He likes to venture out from his place and go into the bush and do certain activities, build cubby houses and that sort of thing. That’s the type of person he is,” Forrestfield Police officer in charge Brad Robinson said.

Police on horseback have helped scour the thick bushland, while drones with thermal-imaging capabilities have searched overhead.

“The aim of the game is to find Finn and bring him home,” Robinson said.

There has been no trace of him so far.

Rainy conditions and cold nights are providing an extra challenge for crews, but police say they are mapping every step in what is a meticulous search.

Police have asked locals to check their sheds and outbuildings for signs of Coyle, “because he could be anywhere”.

“We don’t know whether he’s fallen and injured himself — it could be any scenario,” Robinson said.

“We just want all those people that live around here to have a really good look around their backyards and properties.”

Coyle is described as being 187cm tall with a slim build, short brown hair and green eyes.

If you need help in a crisis, call Lifeline on 13 11 14.

For further information about depression contact beyondblue on 1300224636 or talk to your GP, local health professional or someone you trust.