I know that something is confused here, but cannot figure out what it is myself.
Option #1: If the fifth reportable transaction takes place on September 4 you will file the CTR using that date. You may simultaneously file the DOEP and date it September 4.
Option #2: Should you choose to file the DOEP later, you may use the date you file it.
Option #3: If there is another reportable transaction after September 4 you may date the DOEP as of that later date and file it any time within the following 30 days and avoid filing a CTR on the 6th transaction.