In Lassaigne`s test, which is a confirmatory test for the presence of halogens in an organic compound, the compound is first treated with sodium fusion, which converts the elements into sodium salts. These salts are then dissolved in water and the resulting solution is tested for the presence of specific halogens.

Option 1: Iodine (I) can be detected by Lassaigne`s test. When treated with sodium fusion, organic compounds containing iodine will form sodium iodide, which can be detected.

Option 2: Chlorine (Cl) can be detected by Lassaigne`s test. Organic compounds containing chlorine will form sodium chloride when treated with sodium fusion, and the presence of chlorine can be confirmed.

Option 3: Sulphur (S) can be detected by Lassaigne`s test. Organic compounds containing sulphur will form sodium sulphide when treated with sodium fusion, and the presence of sulphur can be confirmed.

Option 4: Fluorine (F) cannot be detected by Lassaigne`s test. Organic compounds containing fluorine will not form a characteristic sodium salt that can be detected in the test.

Therefore, the correct answer is option 4.