Explanation:
A single-replacement reaction replaces one element for another in a compound.
A double-replacement reaction exchanges the cations (or the anions) of two ionic compounds.
A precipitation reaction is a double-replacement reaction in which one product is a solid precipitate.
Solubility rules are used to predict whether some double-replacement reactions will occur.
The answer is (4), (3) is merely a change in composition, thus not chemical but physical.
Answer: (4).
