I'm no expert but I do know that you can get begin to capture water "movement" at around 1/15 sec. 4" just seems really long. A shorter exposer might help you get the exposure you looking for. I might think that wetting the rocks as others suggested might balace the exposure as well - dunno though.
1/15th did not do it for me, I was going for a very flowing/surreal effect on this one 




