Explanation:

- He added another feather ___ his cap by winning the most valuable player award: This expression means to achieve something noteworthy, adding to one's list of accomplishments.

- Option 1: In: This preposition is not typically used in this idiomatic expression.

- Option 2: Into: This suggests movement inside something, which doesn’t fit the idiom.

- Option 3: To: This is the correct preposition in the idiom "add a feather to one's cap," indicating recognition or achievement.

- Option 4: On: This is not used in the context of this particular idiom.