The period of a wave is the time it takes for one complete cycle. The relationship between frequency (f) and period (T) is:

T = 1/f

Where:

* T is the period in seconds

* f is the frequency in Hertz (Hz)

In this case, the frequency is 4 Hz. Therefore, the period is:

T = 1 / 4 Hz = 0.25 seconds

So, the period of a 4 Hz wave is 0.25 seconds.