Bengali:

One Bengali is a poet.
Two Bengalis is a film society.
Three Bengalis is a political party.
Four Bengalis is two political parties.

Bihari:

One Bihari is Laloo Prasad Yadav.
Two Biharis is a booth-capturing squad.
Three Biharis is a caste killing.
Four Biharis is the entire literate population of Patna.

Punjabi:

One Punjabi is a 100 kg hulk named Pinky.
Two Punjabis is a Pinky with his bigger brother Twinky.
Three Punjabis is an assault on the McAloo Tikkis at the local McDonalds.
Four Punjabis is a combined IQ equal to one.

Mallu:

One Mallu is a coconut stall.
Two Mallus is a boat race.
Three Mallus is a Gulf job racket.
Four Mallus is an oil slick.

UP Bhaiyya:

One UP bhaiyya is a milkman.

Two UP bhaiyyas is a halwai shop.

Three UP bhaiyyas is a fist-fight in the UP assembly.
Four UP bhaiyyas is a mosque-destruction squad.

Gujju:

One Gujju is a share-broker in a Bombay train.
Two Gujjus is a rummy game in a Bombay train.
Three Gujjus is Bombay's noisiest restaurant.
Four Gujjus is a stock market scam.