The frequency is *roughly* proportianal to sqrt(A/VL) where A is the cross-sectional area of the port, V is the volume of the airbox and L is the lengh of the port.
So putting a 4'' tube (of the same length) *ought* to make the intake more efficient in the region of 4-5k rpms.
The Mazda VAD system sounds exactly like what I was thinking of - if there's a commercial application then maybe it makes some difference
Making the main port smaller/longer would shift the factory tuned frequency up/down respectively... the advantage is that with two ports you can have two fillips on the intake efficiency at two different rpms.