What are the four words in which a noun clause can be introduced?

There isn't a fixed set of "four words" that introduce noun clauses. The words used depend on the function of the noun clause in the sentence.

Here's a breakdown of the common ways to introduce noun clauses:

1. Conjunctions: These are the most common type. They are followed by a subject and verb.

* That: *I know that she's coming.*

* Whether/If: *I don't know whether/if he'll be there.*

* How/When/Where/Why/What: *Tell me how you did it.*

2. Interrogative Pronouns: These pronouns are used when the noun clause asks a question.

* Who: *I wonder who will win.*

* What: *Tell me what you want.*

* Which: *Please tell me which one you prefer.*

* Whose: *I don't know whose book this is.*

3. Interrogative Adverbs: These adverbs are used when the noun clause asks a question about time, place, or manner.

* When: *I don't know when he'll arrive.*

* Where: *Can you tell me where he lives?*

* How: *I wonder how she did it.*

* Why: *Do you know why he left?*

Important Notes:

* That is often omitted when introducing a noun clause, especially in informal language.

* If and whether can often be used interchangeably to introduce noun clauses of uncertainty.

Remember, the specific words used to introduce a noun clause depend on the context and meaning of the sentence.