If it was your first grow, I would not expect 8oz on your second. How many watts are you using? Figure that once you get things down, you should be pulling about 1/2 gram per watt. This means that if you are using a 400watt light, you will end up with 7 oz. And this is still kind of pushing it for your second grow. I would figure on close to 4-5 oz dry with the 4 plants. When you are growing more plants, they don't have the same amount of room, they are not all the same height so they don't get the same amount of light, etc. One plant in a cab will yeild more per plant than 4 in the same cab, but the total yeild will be more with the 4 plants.