The term “minimum flow” generally means the lowest continuous flow the pump is permitted to operate, without reference to duration, vibration level or other criteria.
It is the flow below which the pump should not be operated continuously. The piping which supplies minimum flow to pump is known as “minimum flow line”. Normally a piping from pump discharge connected to suction supplies “minimum flow” to pump. It supplies required fluid to pump suction when there is now supply in sump/source (equipment, tank etc.).
Few years back I was assigned trouble shooting job for vibration in minimum flow line. The problem was solved by adding supports in the system (increasing natural frequency of the system).
May be these lines are designed for flow turbulence, that why there is specific requirement of 4Hz.
Point to keep in mind for future projects.
Khalid