Explanation:

- Illustrious means famous, well-known, or highly distinguished.

- Option 1: Doubtful refers to feeling uncertain or unsure about something. It's more about uncertainty than lack of fame.

- Option 2: Sophisticated means having, revealing, or proceeding from a great deal of worldly experience and knowledge. It's not opposite to being famous.

- Option 3: Stormy pertains to turbulent or tempestuous conditions. It is unrelated to the concept of fame.

- Option 4: Obscure means not well-known or hidden, making it the opposite of illustrious.

The opposite of being well-known is being unknown.

Option 4: Obscure is indeed the correct answer.