We can use Faraday's Law of Induction to solve this problem:
EMF = -N * d(phi)/dt
where EMF is the induced electromotive force, N is the number of turns in the loop, and d(phi)/dt is the rate of change of the magnetic flux through the loop.
In this problem, we are given that the induced EMF is 4 V, the magnetic field is 0.25 T, and the time taken is 1.0 s. The magnetic flux through the loop is given by:
phi = B * A
where B is the magnetic field and A is the cross-sectional area of the loop.
Substituting these values into Faraday's Law, we get:
4 = -N * d(phi)/dt
4 = -N * (d/dt)(B * A)
4 = -N * (A * dB/dt)
4 = -N * (0.5 * 0.25)
N = -32
Since we cannot have a negative number of turns, we must take the absolute value of N:
N = |-32| = 32
Therefore, the loop must have 32 turns in order for there to be an induced EMF of 4 V.