F/2.8 lets twice the amount of light hit the sensor that F4 does.
I think we know that.
But what that means to one person is different than the next. For some, its action-stopping sports. I couldn't care less. For me, its a shallower depth of field. I don't think the hand-holdability in dim light is much different -- the weight of the 2.8 make it tougher. At f4, the 4IS is better than the 2.8. Better IQ. Also better at infinity focus at 200mm.
I think another good question would be is the extra stop on the 4IS and 2.8II(IS) worth the extra bulk and weight. Yeah, yeah, TWICE the light-- but again, that means different things to different shooters. I'm sure sports shooters love it. But I'm not sure the stop of isolation (depth of field reduction) is worth it. I never used to question bringing the f4IS, now that I have the 2.8II, I do.