Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
Frequency = (Number of Oscillations)/Time = 40/4 = 10 Hz
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period = 1/(Frequency of Oscillation) = 1/10 = 0.1 sec.