Non-functioning Anti-Shake on my new GRD IV?
I did more tests yesterday, and it seems I was wrong.
In my previous tests, I tried to induce a bit of shake for the test, not by shaking it, but by holding the camera with 3-4 fingers at arm's length. Doing this, the IS didn't help to reduce the shake (see example above).
Yesterday, I did more tests trying to be steady (both hands, etc.) with a lower speed (1/8 or 1/6). And the IS seems to do a pretty good jobs (1 blurry picture out of 10 with IS, versus 5-6 blurry pictures out of 10 without IS).
My conclusion is that GRD IV's IS is good at reducing low-amplitude high-frequency shakes, i.e. the kind of natural shake you induce when you try to be as steady as possible...