f4IS vs 2.8 IS II, simply comes down to size, price, weight, vs one stop of light and use of 2x tc. Beyond that, as for image quality, Nothing between them. just simple s that. Minute sample to sample variations, dof, subject/camera movement, slight misfocus will have far more to do with your 200% pixel peeping analysis than anything else.
Agree 100%. Its a complete draw in terms of picture IQ. Have used both a lot. Have the mrk. II f/2.8 IS L now, because I really need that extra stop. Otherwise I would using the f/4.0 IS L. Was not happy with the original f/2.8 IS L - the mrk. II is visibly sharper - especially @ 200mm where most people tend to shoot most their shots.