Bryan Carnethan at the Digital Picture rated the IS at actually better than 4ev, but testing IS is pretty subject. I am very stable, myself.
I am rating it as "2ev+" for me personally to be on the absolutely safe side when taking a non-repeatable shot. If I wait a little for the IS to settle and am/get very stable myself it is indeed 4ev - but I wouldn't say this what I have come to expect, again for me personally.
When rating an IS system which is all about *statistics* it's also important to make clear what we're talking about: Is it just to still get an average good/usable shot after downsizing, or is it to to get a shot at full mag exactly like one that was taken with a much higher shutter speed?
In my experience, fast shutter speed and IS off beats the IS on the 70-300L in sharpness most of the time, but I feel safe shooting with 1/250 x-sync at 300mm on crop (i.e. 480mm ff) and can expect get near-optimal iq at 100% mag. Without IS I'd feel safe with 1/1000s, so it's a 2ev advantage - remember the old 1/focal length from the ol' analog days doesn't really apply here because nowadays the sensor resolution is so high.