Small discus do better in a group. AS with almost all cichlids they have a pecking order. If you only have 3 or 4 the strongest will pick on the weaker ones until they get sick and die. Normally 6 is the smallest group that you can get a way with and have success. It helps spread out the aggression. 8 to 10 works much better.
Godd Luck.
-john
P.S this was written in reference to growing out young fish. Adults can be different. Then it just depends on the fish but normally larger groups still do better.