[MysticLlama]

Air volume flow rate (qv)
The volume flow rate is obtained by multiplying the average velocity (v) by the cross sectional area (A) where the velocity is measured. It is expressed in cubic metres/second (m3/s).

qv=v * A

I'm going to make average velocity 100mph to make the math a little easier.

Since that has to be expressed in cubic meters, we'd have to assume to attain the velocity measurment, it has to be a square meter since I used the index of M/sec to MPH.

A 4" circle is about 12 square inches. This is about .0075 square meters.

Index to convert mph to m/sec is .447, so this would make the velocity @ 100mph in (m3/s) 44.7 given a 1sq meter area.

qv=44.7 * .0075

.33525 = 224 * .0075

.33525 cubic meters = 11.84 cubic feet/sec

So it looks like a 4" hole should produce about 710CFM at 100mph?

This might be 150% wrong, I haven't done math like this in a LONG time. I just kinda made it up as I went and looked up some formulas / conversions.