It will base on when on the approval date, whether the 2004 remaining quota reserved for the cases in queue is up or not.
If your case will be adjudicated very soon, or you have used premium processing, 2004 cap is still available for you. If you case is slow, or another RFE is sent, and the cap reserved for the pending cases is up and then you will be using 2005 cap after Oct 1.