Even with high ISO's the fact that you hae less light coming in at F4 then at F2.8 it will effect the autofocus system in the dimmer situations.
Another factor is the longer the focal length the faster you require for a shutter speed on a moving subject. ISO can help get you there but image quality will suffer.
The VR is useful is only certain situations and moving subjects is not one of them. Typically the longest focal lengths are being used for moving subjects such as sports, Motorsports, wildlife, birding etc.
Also think about subject isolation. The F2.8 will isolate more then the F4. Thanks for doing the test though. Truth is, thee who need the F2.8 probably already know and those who need even faster buy a 200 F2