Explanation:

- Option 1: such a

- "Such a" is used correctly to express the degree of something. The phrase "such a" fits well here, describing the kind of clever boy being discussed.

- Option 2: that you are

- This part of the sentence is the source of the grammatical error. "That you are" suggests a comparison but doesn't fit correctly here. A more appropriate construction would be "as you are" to express equality in qualities.

- This option contains the error.

- Option 3: clever boy

- The phrase "clever boy" is grammatically correct. It accurately describes the boy's attribute.

- Option 4: He is not

- "He is not" is grammatically fine. It correctly negates the statement following it.