With only 4 fish you may be seeing some normal fighting for pecking order dominance. The smaller one sounds stressed and it may not be eating for that reason, or perhaps has some other health issue. Discus are social, schooling fish and it is usually recommended to keep them in groups of 6 or more to keep down the fighting. You also want to be careful about contaminating your older fish by adding new fish that could be carrying something. Quarantining new fish for up to 6 weeks is usually recommended. Without pics or more information it's hard to know for sure what's going on. If you continue to have problems you might want to post in the emergency room section and fill out the questionnaire.