Badger, try holding that down shutter down for a long time... you'll see the buffer gets filled quickly. So yeah, it's 3.9 fps for a while until the internal buffer fills. Then you have to wait for the buffer to clear (i.e., write to the card). A faster card can't hurt (except in the wallet), but I don't know how much it would improve the overall buffer situation.
For the newer cameras I think it would make a noticeable difference. Then again, I'm shooting with the 60 mb/s cards like you.