new cat fighting my other cats

I just got a new foster cat and brought him to meet my other 3. They have their own cat room and my cats have gotten along with all my other fosters but this one. He doesnt seem aggresive but 2 of mine were bullying him a bit and not accepting him. Now he attacks my other cat that wasnt bullying him, then the other 2 surround him and growl, hiss and stare him down. What can I do to calm them??

Give them about 10 days... seems to be the usual time it takes. Others here will suggest separation for a time, exchanging smells on towels, feeding routines, etc., crating, but I've never bothered with any of that (it honestly doesn't seem to make a difference anyhow) and they all seem to cool off in about 10 days.

Thanks. They never really had a big problem with my other fosters. The usual whacking and hissing but never fighting. I just wander what it is about him they dont like. Ill wait it out, I dont want to take him back to the shelter. Wish him luck on finding a real home please!

The scale of aggression would determine if I would separate them or not. If it's just hissing perhaps you can get away without the separation. If it's out and out physical combat I would definitely separate them and have monitored get-togethers. If the physical combat started during the get-together, I'd quickly separate again and keep doing that on a regular basis.