If I see a recipe calling for 4 ounces of ap flour, am I free to combine a few flours to make 4 ounces (since I'm not going by cups)?

It depends what you are making and the type of flours you are going to use. Frankly, I would buy all purpose flour and not try to mess with the recipe. You need to approximate the protein and gluten levels, texture, etc. that the recipe requires.