Four gunmen sledge-hammered their way into a Wells Fargo depot, overpowered guards arriving for work yesterday and stole $8 million in cash from a vault. They fled in an armored car, leaving $12 million behind.

The robbers, three wearing ski masks and the other with a turban over his face, knocked through the cinder-block wall of an adjacent building into the Wells Fargo Armored Service Corp. depot in lower Manhattan, Chief of Detectives Richard Nicastro said.

The depot had been without security guards since 2 p.m. Sunday, he said.

When four armed Wells Fargo guards reported for work at 1:30 a.m. yesterday, the robbers -- watching through the video security system -- surprised them as they opened the vault for a routine inspection.

The guards were disarmed and handcuffed but not harmed. The robbers then loaded the cash onto a Wells Fargo armored truck and escaped, Nicastro said.

The truck was found empty under the Brooklyn Bridge 1 1/2 miles from the depot about four hours later.

Nicastro said the bandits, armed with revolvers, were calm and loaded the truck in 15 minutes.

Part of the robbery was videotaped. Nicastro said at least one member of the gang must have known the layout of the Wells Fargo building.

Reports of the amount stolen were steadily scaled down -- from $50 million to $25 million to $8 million.

The depot is used in part as an overnight depository for cash going from banks to the Federal Reserve bank. The FBI said the loot was "spending money" -- meaning that no checks or other bank notes were involved.

In 1983, $7 million was stolen from Wells Fargo in Connecticut; in 1982, $11 million was stolen from Sentry Armored Car-Courier in New York.