How much concrete for a fence post?
Wow! I normally use post hole diggers (6" diameter?) 3 feet deep and ½ a bag of redi-mix per post. I did a free standing section of 6 foot privacy fencing this last summer and we just had a storm of >50mph winds and it is still standing and solid.

So, some of it depends on the quality of your soil but 4 bags per post would be major overbuilding in my opinion.

If you had access to an 8" auger and a full bag per post would still be overbuilding but if you are OCD then that would probably satisfy your worries.

Good luck!