I would not put 4 in as i never keep even numbers of discus, it's to do with the pecking order. Your tank is not big eneough for 5 so my answer would be to keep 3 fish, incidently all my discus are for sale, pick up only Leeds. I would put some corydoras in the tank but only the one will stand the higher temperatures of a discus aquarium. Your tank should be properly cycled and your water aged and if this is the first time keeping discus then please read everything you can before buying any. A few tetras as dither fish would also be a good idea, i have some of those for sale too lol.