Of course you can do 4fps. With 8fps each shot takes 1/8s, adding 1/8 antishock cuts frame rate in half. Since E-P5 shoots at 9fps it will be a bit faster. I didn't measure, but it sounds like that, faster than 3fps on E-P3.
The only limitation is that antishock adds to shutter lag for the first shot.
It's actually a useful way to get sharp shots from bursts when IBIS is at its limit.
Is C-AF still available when doing this?