BTW, all cats are healthy and are vet checked regularly.

Do you have enough litter boxes for the 4 of them (should have at least 4)
and are they each getting their own food dishes? I found this happened to
me, also, when I added a fourth cat to my household last year. I would
always break up the fights. Also, do you have plenty of room vertically for
them (ie. window seats, etc).
Gail

When I had a bully male cat to deal with, I did not know what to do so
I did something which is probably not all that great. I let the bully
know if he messes with the smaller cat, he will have to deal with me. I
would pick up the bully and bully him. He became aware of me. He no
longer attacked the other smaller cat. He would look at me first :)

If I saw the cat stalking the other cat, depending on my strength, I
would pick the cat up by the scruff of its neck, if I could, look it in
the eyes, and say firmly and steadily, No. Got it? No! The bully cat
above was too big to pick up by the scruff of its neck so I picked him
up by his chest. He was a big, mean cat who attacked everybody and
everything. He was go-go-go all the time.

I am sure there are more sophisticated measures, like "Clicker
Training" to distract the cat. You might want to ask in cat groups
where there are behaviorists and others who can finesse the situation.
I only did what I mentioned above because I did not have the time and
the bully cat was not my cat. I can't say my cat was all that innocent
since she attacked the bully cat first when he trespassed into our
territory, but that's another story.