Period of one oscillation of a PWM signal at 4Mhz and a counter
The PWM period is the clock period (\$2.5×10^{-7}=250\text{ ns}\$) multiplied by \$2^n\$, where \$n\$ is the number of bits resolution.

A 16-bit PWM with a 4MHz clock would have a period of 16.384ms.