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.
Counterpoint: There will be much more work to be done in socialism and communism than in capitalism, because our cultural standards will be much, much higher; a socialist world is one where everyone will have first-world problems. Just like living in the cramped and damp huts of the dark ages seems unthinkable today, it will be inconceivable to socialist society to live in an empty room painted eggshell. People tomorrow will not be content with derivative sequels and machine-made mince music, and they won’t wear pants that can’t survive the month. To support billions of people with luxury quality that has low environmental impact is a thing that can and will be made possible, but it requires much more work than supporting a deadbeat proletariat that is economically clinging on for dear life and teetering on the brink of ecological extinction.