The formula for acceleration due to gravity can be calculated using the formula:
g= G m / r^2
where g is gravity, G is gravitational constant, m is mass, r is radius
So we can see that if we increase m by 4, so g also increase by 4. But if the radius is decreased to ¼, therefore the gravity will be divided by (1/4)^2. So:
4 / (1/4)^2 = 64
So the gravity on the surface will be:
64 g
(64 times that of Earth’s gravity)