i'm very curious about that. i'm wondering if the 400pf will offer advantages over the 100-400
It almost certainly will be a bit sharper, as tends to be the case with high quality single focal-length lenses vs. zooms. The difference in most situations might encourage mostly nitpicking by pixel peepers, but it's very likely reality, and all other things remaining equal (and of course they never are), most people prefer sharper to not quite as sharp.