This poor cat isn't just angry, she's scared and confused. She went from her own familiar home (and person) to a new home, then back to her person, and now to you, in a strange home with 3 strange cats. Pretty scary for any cat. It will take her awhile to feel safe and secure, several months or more. Sharing your office with her is a great idea. If there's not already a litter box in there, add one and keep her in that room only, separated from the other cats and the rest of the house. She'll be much happier and feel much more secure in one room. A new home can be very overwhelming and she's already so stressed. Having a room of her own with things that are familiar to her, along with regular visits from you, should go a long way in calming her. Be patient; don't push her; don't approach her or give her any reason to get upset. Let her come to you, and she will in time. Talking quietly to her is wonderful and will begin to soothe her even more as she grows accustomed to your voice and your company. If she likes treats, you can bring her a few treats each time you visit her, and she'll begin to look forward to it. Once she's settled down a bit, which she should in a few days, play with her with a string or fishing-pole type of toy. She needs to run and jump and chase something to burn off some energy. She's a very young cat, energetic and frustrated. Spaying her will help her become less aggressive.
After a week or so, start feeding your cats right outside the office door. If your 3 won't eat together in a small space, then take turns with them. The point of this is to let them smell each other while they're doing something enjoyable like eating. Take slightly damp washcloths or small towels and swap their scents -- rubbing them on your cats and then on the new one, then on the new one first and then on yours. Since smell is the most important thing to a cat, you can help them become familiar with each other by exchanging their scents. You can also take a blanket or cat bed that the new one has slept on for a few days and put it out in a room where your cats can sniff it and maybe even sleep in it. Do the same for the new one by bringing in a blanket or bed that some of your cats have used. And continue feeding them on opposite sides of the door. After a couple of weeks, open the office door a crack now and then when one of your cats is around. They can sniff each other (and probably hiss) and add to their familiarity. Keep the sniff sessions brief at first so they don't become growling/swatting opportunities. Eventually you'll get to the point where you're letting the new cat out into the house briefly and then for longer periods of time. Have a squirt/spray bottle around with water in it and be prepared to spray if the hostility gets out of hand at all. No swatting or attacking allowed. Have treats with you and be sure to praise and treat any cat who's being tolerant. If this all sounds time-consuming, well, it is. But each of these things take no more than a few minutes at a time. Be consistent and stick with it. This process won't create best friends, but what it does that's so important is that it gives the introduction process some structure and guidelines, which does so much in terms of getting the relationships off to a good start. You're already doing a number of positive things! Your friend is very fortunate that you're willing to take in her cat. When a cat behaves aggressively in a shelter situation it is usually euthanized. Shelters generally don't have the time (or the staff) to be patient and work with a frightened cat. You are literally saving this kitty's life.
owned by cats 40+ years; shelter cat volunteer; foster mom for kittens