What is the final speed if the displacement is increased by a factor of 4?

From the laws of motions:
x = 0.5 at^2 where
x is the displacement
a is the force of gravity (constant = 9.8 m/sec^2)
t is the time taken

Since "a" is constant, therefore:
the displacement is directly proportional to the square of the time.
This means that, increasing the displacement by a factor of 4 would increase the time by a factor of (4)^2 = 16.