I'm guessing Bitmain literally means before September ends, like literally shipping on the 31st.
My uneducated guess is they're waiting for any BTC price pump up, so they can better profit on the S4 sales, and mitigate shipping costs.
Well, if we have to wait until Sept 31...wait a minute, there is NO Sept 31

LOL, you caught that didn't you?

Yea, they're shipping this thing in October it looks like.