Don't worry. 1 week is a relatively short time. Since he is 1 cat going into the territory of 3 resident cats, he is at the dissadvantage. Make sure he has a safe place that he can go to get away when needed. I always start my new additions (and that is what he is to this house) in a room alone and away from the resident cats. He will have a chance to learn his new environment a little before he needs to meet his new room mates. Then introducing 1 cat at a time to him starting with the one who will be the least threatened and the least threatening to him. Good luck, keep us posted and ask more questions if you need to.