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Hello there. My name isXXXXX am sorry for your dilemma. I think you should try to move all of them together so not just the one is left behind. That would be very stressful on her especially since she enjoys interacting with the other one. You can use a live trap which is just a big wire cage that when she enters it triggers the door behind her. you can ask your vet if they have one you can borrow or you can buy one at most major pet stores. You would just relocate the ones you can touch into a room so they don't go in the trap. Then put food in the trap and pick up all over food. She will eventually (may take a day or two) go in there and trigger the trap and you can then either gently dump her into a carrier or just move her in the trap. Its safe and does not harm her at all. Or you can ask your vet for a tranquilizer in pill form, hide it in a nice tasty treat and give it to her it will take a couple hours but then she should be dopey enough that you can place her in a carrier to transport her. If these options do not work then the second option is to leave her behind, hopefully since she will be alone the stress will be minimum and your husband can work daily on getting her to warm up to him. Just by spending time daily talking to her and giving her treats. Since the other cats are gone then he can focus on her at that time and hopefully eventually get her in the carrier or again he can do the live trap plan or the tranquilizer plan.
I am just afraid that if you take the three other cats away it may make her revert to being even more scared and stressed so it would be ideal if you can move them all together. The other fear is moving the three and then bringing the 4th 8 months later. .. they may see her as an outsider again and you will have to reintroduce them all over
So in review I would try either the tranquilizer or the live trap to catch her and relocate her with the other three cats at the same time. Then if that does not work relocate the other three cats and have your husband work on socializing Sally and getting her to be comfortable enough that he can get her to go in a carrier or worst case scenario end up having to do the live trap or tranquilizer in this case.
I hope this helps and I know it is stressful but you are doing a great thing and they are all very lucky to have an owner who cares this much!
Please let me know if you have any additional questions or concerns. I would be happy to help. Please do not forget to rate my response so I receive credit for my time. Thank you!