A man goes to a fair and buys some 81 gold coins. After he pays for it, the guy who sold those coins tells it to him tat one of those coins is a fake and it weighs a bit less than other coins. This man comes home n wants to find which one is the counterfeit one. He is puzzled how to find the fake coin easily n the only thing he has is a small balance scale with 2 pans hanging on the 2 sides. His servant comes along n the man tells him his problem.

The servant smiles n says , "Sir i can do this with jus 4 weighings on the balance scale n tell u which is the fake one n tell u which r the 80 good ones" and he does as he says.

How does he do so ?

(Hint : This is a purely logical qsn and is not a googly. so plz think only on how he weighs them )