My cat is bullying a younger cat. Help!

I have 4 cats, 2 are only a year old. But my oldest cat (7yrs) keeps chasing the youngest out the room, making him extremely timid. I think it's territorial behaviour, she's not like it with the others. She's become so aggressive that when I tried to comfort the scared cat the oldest attacked me. I've tried feliway sprays, what else can I do?