The appropriate preposition to use in this context is "below."
The sentence indicates that the temperature reaches a point that is 4°C *lower* than some implied starting point.
"From" (1) would be more appropriate if we were specifying the starting temperature (e.g., "The temperature falls from 10°C to 6°C").
"At" (3) refers to a specific point, not a range or change. "Into" (4) is used for movement or transition, not temperature change. Conclusion:
"Below" is the correct preposition to indicate the temperature falling to a point 4°C lower.