Explanation:

- Option 1: A well-wisher

- This option implies someone who has good intentions and wishes for your success. It depicts a positive and supportive relationship.

- Option 2: Difficult to find

- This suggests something that is hidden or hard to locate, which isn't the essence of the idiom.

- Option 3: A good friend

- Similar to a well-wisher, this indicates a positive and trustworthy relationship, not the deceit that the idiom conveys.

- Option 4: A secret enemy

- The idiom “a snake in the grass” refers to a deceitful or treacherous person who conceals their true intentions, much like a snake hiding before it strikes.