I would still stick w/ the 1/FL rule. Some may even go as far as 1/(FL * crop factor). I only dip down below that when it's absolutely necessary. Yes, IS is great but why let it be a crutch.
That said, stationary subjects + good technique, @ 400mm, you can probably get away with good keeper rates at 1/100s. But if the subject moves and or you've got a bad caffeine tic... good luck.