A footprint believed to belong to missing four-year-old boy August “Gus” Lamont has sparked fresh hope in the desperate search which now entering its fifth day — but police warn time is running out.

The print was discovered on Tuesday night, just 500m from where the little boy was last seen playing outside his family’s remote sheep station, 40km south of Yunta in South Australia’s Mid North region.

The alarm was raised just after 5pm on Saturday, sparking a massive search across the isolated 60sqkm property surrounded by outback-like conditions and dense scrub.

“We’re buoyed by that fact,” Superintendent Mark Syrus said in confirming the footprint matched the boots Gus was wearing when he vanished.

Despite the breakthrough, Gus has now been without food or water for days.

“Our priority has always been to find Gus alive,” Syrus said.

“But he’s been gone for 86 hours (at the time) — no water, no food, no shelter. The chance of finding him alive is really diminishing every minute.”

Forty police cadets have travelled from Adelaide to join the search, which now includes a specialist tracker with deep ties to the land.

The cadets join more than 100 personnel including police officers, SES crews and volunteers, plus drones, helicopters, dogs, horses, police divers and aircraft scouring the vast outback property.

On Wednesday the Australian Defence Force confirmed to 7NEWS the army has been called in to support the search.

A bootprint believed to belong to Gus was found on Tuesday evening Credit: SAPOL

Police have spoken with Gus’s family to prepare them for the possibility he may not survive.

“We always remain hopeful that Gus is a tough little country lad, curled up under a bush somewhere — and we’ll find him today,” Syrus said.

Gus is Caucasian with long blonde curly hair.

He was last seen wearing a grey sun hat, cobalt blue long-sleeve shirt with a yellow Minion on the front, light grey pants and boots.

Gus was last seen a Minion long sleeved T-shirt before he went missing on Saturday. Credit: SAPOL