You shouldn't count iOS5 IMO, 4 is going to get iOS5 too most likely.
I don't know what could the killer feature be, I mean it's going to be more powerful, 64GB, dual core, better camera ... but that's expected and evolutionary, the screen will stay the same, other than that, NFC maybe? But NFC is definitely not a killer feature to me.
iPhone 4 has pretty much everything, great screen, BT, GPS, Gyroscope, Magnetometer, Accelerometer, etc... iPhone 5 will be more powerful but I doubt it'll have any new features on the hardware side.