half
Reason — Time period of a simple pendulum is given by:
T = 2π \sqrt {l\over g }
Time period is directly proportional to the square root of the length of the pendulum.
In the case when length is made one-fourth, we see that —
T = 2π\sqrt {l\over 4 \times g }
T = {2\over 2} \pi \sqrt {l\over g }
T = \pi \sqrt {l\over g }
Hence, we can say that when the length is made four times, time period of a simple pendulum is reduced to half.