Cannot get new cat to stop attacking ANY other cat

About 4 months ago, I adopted a new male Siamese cat (named Smudge), who I recently got neutered and is around 1 years old. I have three indoor cats which all came into my house during different times who get along swimmingly. I've introduced adult cats to other adult cats before and know it is a long process to get them to tolerate each other. I gave smudge his own room for a few months, switched that room with the other cats, then back again a few times so that they all can get familiar with his scent. I had them feed on opposite sides of a door for months as well. I know the other cats and Smudge recognize his scent and vice versa.

ANY time Smudge comes in contact with the other cats, he IMMEDIATELY attacks them without warning. No vocalization, no hissing, growling, etc. I've never seen anything like this before. It gives the other cats no opportunity to defend him/herself or challenge the threat. They are totally caught off guard and it always ends violently. It's actually quite terrifying to watch.

Smudge is very good to humans, never has scratched or bit me. He likes human interaction and is very affectionate. I would think that after 4 months and constant "scent swapping," he would know that the other cats are not a threat. I don't know if this is a lost cause and has been too ingrained in his behavior to change. There is no "middle ground" with him to work on and I have no basis to start coaching from. The other cats are so traumatized of him that they cower in fear from just the scent of him. Help please!

Wowzer, 'A' for effort indeed. 

You've certainly gone to bat for him and your cowering unfortunate trio.

We need a cat whisperer like Caesar is to dogs. You have a very tough scenario.

Personally, I would remove him from your dwelling via a posting on PetFinder, Craigslist, shelter etc.  Know anyone that wants a loving 1 cat ONLY household?  Considering he's had ample time to smell them, obviously he's not keen on any feline company.   

Do you have a screened in section, that they could all look and smell each other, but not have contact? That might be a final test, not sure.

 Marc:  

Thank you SO much for your reply. I do have a "catio" which I've put him in. The other cats run from the sight of him in there. I kindly brought one of the other cats to meet him there in my arms, but Smudge started trying to get out to pounce on him. It made the other cat run away. They've been traumatized. I think too much damage has been done to go back.

I really think I will take your advice and try to find him a loving home where he can get the sole attention he deserves. There may have been something in his past that conditioned him to act in such a manner. I don't think I have the expertise to condition him in such a drastic manner.