There is an English word that can be used up to four times in a row without modifying the spelling and form a valid grammatical sentence.

Do you know what word is that?

The word is "Buffalo".

According to the dictionary meaning, Buffalo is a noun meaning an animal. It is also a verb meaning "to bully".

Sentence: Buffalo.
Meaning: Go and bully someone.

Sentence: Buffalo buffalo.
Meaning: Animals bully.

Sentence: Buffalo buffalo buffalo.
Meaning: Animals bully other animals.

Sentence: Buffalo buffalo buffalo buffalo.
Meaning: Animals that are bullied by other animals, in turn, bully other animals.