Think of light spreading out from a bulb; as you double your distance from the lamp, the light intensity drops to one-fourth due to the inverse-square law.

Inverse Square Rule: Double the distance, divide the force by four!

Coulomb's law is expressed mathematically as $F = \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2}$. Because the force $F$ is inversely proportional to the square of the separation distance ($F \propto \frac{1}{r^2}$), substituting $2r$ for $r$ yields $F' = \frac{1}{(2r)^2} = \frac{1}{4r^2}$, which is one-fourth of the original force.

Keep in mind that Newton's Law of Gravitation follows the exact same inverse-square dependence on distance, so distance modifications affect gravitational and electrostatic forces identically.