You haven't stated the standard deviation or provided any more information. If the standard deviation is s then, with 3.9 as the mean, 3.9-2s is 2 standard deviations below the mean.
For example, if s=0.4, then 3.9-2s=3.1. But if s=0.9, then 3.9-2s=2.1.
Maybe you meant what is the probability of a value being 2 standard deviations below the mean, that is, the Z-score is -2. The answer would be 0.02275 or about 2.3%.
