As tough as it is, if bringing your old lady into the house truly isn’t an option, I would really consider putting her down before moving in four new cats. Spoil her silly, give her all of her favorite treats, and let her go in her barn without interlopers. Having a whole crew move in is just so likely to be so very stressful for her, and that really isn’t fair to her at the end of her life.
I’m sorry, it’s always so hard when they get older. Cancer is such a beast.