One of my rats is a bully-experienced rat owners only please?

I have four rats, and three of them are very friendly and playful and always get along and enjoy getting held. But on of them (which I found from a newspaper ad) is a horrible bully and beats the living sh*t out of the others. I have two large cages, so I keep the three separated from the really big mean one. I've tried to put them in a different area together so maybe it won't be so territorial, but this one just seems to be out for blood. If I try to put any others in his cage they get super freaked out because he's immediately on them once they set foot in the cage. This rats also really skittish, however he's gotten used to me and lets me hold him and play with him. I''ve found he can actually be very sweet and cute, he's just misunderstood (lol, cheesy, but true). I don't know what to do because I know if I give this rat away he's definitely going to be used as a feeder rat since he's so unfriendly and I love this little guy and I feel like we have a bond, but at the same time I feel so bad for him because he has to sit in that big cage all by himself while the other ones all have each other. Not to mention, it seems unfair that he has one big cage to himself while the other three have to share theirs (since I originally planned don combining the two cages). Anyways, does anyone have some advice and have you dealt with a bully rat before?