Yes, you would need to purchase a minimum of a 4 day ticket with park hopper option to qualify for the FD offer.
If you purchase APs in September and go back while your APs are still valid, you can go to guest services and ask them to put your 4 day passes on your account after your APs so when you enter the parks you are not using up your 4 day passes.
Just a heads up that the 4 day passes will have an expiry date. If you don't end up using them before the expiry date they are still valid and worth the amount you paid, you would just need to pay the difference between what you paid and what the value of the passes are at the time you go to use them.
Just a thought, before you upgrade to APs you may wish to run your numbers. My DD and I went to Disney 3 times in a 1 year period and even though those trips included 19 days in the parks, it was still a better deal for us to keep the tickets as they were as 2/3 of those trips were
Free Dining trips and it was a $400 savings for us to pay for the park passes and get the free dining.