If it were me, I'd stop at 3. You might get away with 4 but then your levels could suffer as well as fish health. Instead you could add corals and critters. I have a 10G set up with the only fish being a watchman goby. But in with it I have some coral, a pistol shrimp, a pepperment shrimp and different types of hermits for color. Just because you're out of room for fish, doesn't mean you can't add other things. Just be careful when choosing. Don't pick an invert that starts small and gets big. I bought a brittle star at about 2" in length. I failed to realize it was a green which get huge! Luckily I have a 180G which he is now in and has grown to around 15+"! Also some(mostly stars) take a certain amount of nurishment from the sand bed. In a small tank, most of the time they can't get it.