Explanation:

- The original sentence states: "I went to the market buying few vegetables and milk."

- Option 1: "to buy few" – This option uses "few" without a qualifier, which often implies a smaller quantity than intended unless used for negation or contrast.

- Option 2: "No substitution required" – The sentence needs alteration for grammatical clarity and quantity context, so no substitution is inaccurate.

- Option 3: "buying some" – Similar to the original, it doesn't clarify purpose or completion of action as effectively as an infinitive form.

- Option 4: "to buy some" – This provides the correct intent using the infinitive "to buy" and "some" implies an indefinite yet sufficient quantity.