F = GMm/r²
The relationship between F and r is an inverse one. So if the distance r decreases, to 1/4 the original distance:
Let F α 1/r²
F = k/r²
or F₂ / F₁ = (r₁ / r₂)²
If r is decreased to 1/4,
F₂ / F₁ = (r₁ / r₂)²
F₂ / F₁ = (r₁ / (1/4r₁) )²
F₂ / F₁ = (4)²
F₂ / F₁ = 16
F₂ = 16F₁
The new force of attraction would be 16 times the former value.