At what temperature and how long would I cook brownies or little cakes?

I usually bake mine at 325, and depending on what you're baking, they take anywhere from 12 to 20 min to be done. It really depends on how moist your batter is and what you're making. I just made oatmeal cookie bars with mine and they took about 17 min to bake, but that was with added banana and other stuff. Hope this helps, best of luck with your baking!

How full to fill each cavity?

Fill Half way- when its done baking it will be at the top of the opening. It balances out perfectly.

I worry these might be too big for a 4qt square air fryer. Has anyone actually tried these in a 4Qt or mini vortex?

8.3 inches by 5.2 inches. Too big for my 5-quart air fryer so probably also too big for your 4 qt. However since it is silicone you could theoretically cut off the top two compartments with scissors and have the smaller 4 compartments that could work. Unfortunately, that would destroy the thicker outside rims possibly forcing you to fill it with batter while being stabilized inside the fryer compartment. Even with the rims all silicone bakeware is kind of floppy.