I cant find anything on the Waves site that says anything about C4's ratio. Maybe this could be a good thread to start up a sound comparison test using the same settings on each of the different plugs.
Say, record a clip and process it with the settings using C4 and then using the same settings with the other multiband, render a few clips adjusting the ratio parameter different on each one.
It'd be a good idea to include a poll and get everyone to vote on which one they think sounds the most like original.
Its not as easy as "4.5 dude" but its something to do, I guess.
Hopefully this helps.
C4 doesn't use the typical ratio setting. It uses "range". It's a different concept than ratio that I don't fully understand (though I've only read a few sentences about it and haven't researched it further then that). I just end up playing it by ear. Try goggling about the range settings, maybe there's something out there (I haven't looked either).
Range is basically the maximum amount it will compress. If range is set to -5, c4 will not compress more than 5 db.
So if you want to translate range the best thing would be to experiment with the ratio until you compress the same amount as the range is set to as maximum.
On top of that, changing the 'Knee' setting affects the ratio. Also:
In the C4, the Threshold will be the centre of the RANGE value; that is, if RANGE is 8dB then the Threshold marks the
4dB gain reduction point (youll have 4dB of Gain Reduction if the signal is equal to the Threshold).