4 ounces is the smallest amount you can buy and is more than enough to do what you want. I think the last receiver set and handguard I did took about 40ml. The Cerakote book says to use 72ml for a rifle and that has been too much in my experience. It also depends on the color as some cover better than others and will require fewer coats.
Just for reference, there is 118ml in 4oz.