Memes

The question is a bit misleading, since it is not about relative acceleration but relative velocity. The relative velocity of the 4-year-old man is key to determine his momentum, and hence the kinetic energy of his impact upon the bullet.

With that out of the way, we first note that adulthood starts at 18, which must be due to a significant time dilation in the reference frame of the man. We have the formula for the time dilation t' = γt, with the Lorentzian gamma factor γ = 1/sqrt(1 - v²/c²), thus 1/γ² = 1 - v²/c², and we get v = sqrt(1 - 1/γ²)c for the velocity. If the man is four years old in one reference frame and 18 in another, then γ = 18/4 = 4.5, and after plugging in the value, it follows that v = 0.975c. Therefore, the man had an incredible speed of about 292500 m/s when he and the bullet mutually obliterated one another.