I know there are many different motors and gearboxes out there, but here's some hard data on Tamiya gearboxes and motors:
Actual RPM's were measured with a tachometer and the gearbox ratio was determined by actual manual rotation count.

Four stock Tamiya motors were measured. All were almost exactly the same RPM. Average [email protected] no-load: 16,600 RPM
An ETO "Long-black" upgrade motor was also measured. [email protected] no-load: 17,300 RPM

A standard Tamiya gearbox (measured from my KT and T1) with a standard 10-tooth motor pinion has a ratio of 60:1 (60 motor revs = 1 wheel rev)
That calculates to 266 wheel RPM at full no-load motor speed of 16,600 RPM.
266 wheel RPM calculates to a scale speed of 26 MPH (for a Tam KT and T1, each with 2" diameter drive wheels).
My actual KT top-speed measurement comes out at 22 scale MPH, slightly slower than the calculated 26 MPH.
The slower speed is due to the slower maximum motor speed under load vs the higher no-load RPM.

Note: the Tam gearbox ratio, not including the motor pinion (10 tooth) is 600:1.

So all this 4:1 and 3:1 nomenclature really lends confusion, since the gearbox ratios are actually up in the 600:1 range.
The 3 and 4 shaft count description is one way to generally describe gearboxes, but it's the number of gears on each shaft and their tooth-count that determines gear ratio, not just the shaft count.
So neither method, shaft count metod or 3:1/4:1 method, are accurate ways to describe the actual gearbox ratios.
I can provide example calculations if wanted.