Yup, a 4 foot tank is enough for one Oscar as an adult, so as it is you are going to need to get rid of two fish. If you are luck a compatable pair will form and you might get away with two. Hopefully your tank has a depth of 18", not 12"?
If you want to keep just one Oscar then you can get a large schooling fish that can't be considered as food. Silver dollars for example, could work. There are never any guarantees with cichlids, only ideas.
While you technically can house other cichlids with them, I don't recommend it. A bunch of aggressive fish in a tank is a popular route to take, but it is hardly the best situation to have - a bunch of aggessive fish in a tank, always looking over their shoulders, under constant stress. They can live like that, but if you want what's best for the fish it's best to keep them in species tanks, unless you have a huge aquarium.