"An irate employee breaks into his boss’s locked file cabinet, steals an incriminating letter that bears his employer’s signature, and takes it to the police. Has there been a search within the meaning of the Fourth Amendment? May the police use the stolen letter as evidence?"

Yes, They can.

Explanation:

The fourth amendment protected every citizens in united states from unreasonable searches and seizures. This means that if police want to search for evidence from private property, the search need to be conducted with a warrant.

But, in the case above, police was not the one that took the evidence. It was another individual who later gave the evidence to the police.

So technically, the police wasn't conducting any search that protected by the fourth amendment.

im sorry i wish i can but try google