Seeking "cat-whisperer" advice ---- RE: New Cat in the house, compatibility with my older cat, etc

[Curmudgeoness]

2. I have done this a few times.

It has never been the ideal that I would have hoped, but it is also never as bad as I feared. I would say that 4 days is long enough to keep them separated. What I do is leave the "safe haven" of the new cat's room as it is, but just leave the door open. The new cat can investigate the rest of the house in its own time, or your cat can check out the new cat in it's room. The new cat can still use the room as a safe place to retreat if necessary.

My current cats have been together since December, and they are still not friends. There is still a lot of hissing if they get close to each other, but there has never been more than noise. I have come to the conclusion that they will never be friends, but they are usually fine with just ignoring each other. Then again, both of my cats are rather dominant. Hopefully, your kitties do better, but it isn't too bad if they are never close.