Each option in this question refers to a different statement related to chemistry.

Option 1 states that Sodium Chloride (NaCl) enhances the setting rate of Plaster of Paris. This is correct because NaCl acts as an accelerator, speeding up the rate at which the plaster sets.

Option 2 states that Gypsum is added to cement to slow down its rate of setting. This is also true. Gypsum regulates the setting time of cement, preventing it from setting too quickly, hence making it more workable.

Option 3 states that all alkaline earth metals form hydrated salts. This is generally true as alkaline earth metals often form hydrated salts.

Option 4, the correct answer, states Barium and Strontium are found free in nature. This is incorrect as neither Barium nor Strontium is found in free state in nature due to their high reactivity. They are usually found as compounds.