The heaviest 4% of the fruits corresponds to the 96th percentile.
First find the z-score that corresponds to the 96th percentile, which is 1.751 as P(Z < 1.751) = 0.96.
standardized z score z = (x - mean)/standard deviation, where x = fruit weight
1.751 = (x - 415)/31
solve for x to get fruit weight for 96th percentile (or lower bound of heaviest 4% of fruit)