There is much debate on the difference between .999 and .9999 silver, .9999 is ideal and .999 is the minimum from most literature I have read.
From the literature I have read, the difference between .999 and .9999 isn't sufficient to worry about. One study showed that the water, even distilled is likely to contain more impurities than the silver at .999.
So you might want to worry about your waters purity than the difference between the two.
Of course if I was selling .9999 silver I would be hysterical and try to scare you into buying my product.