Hi Pink chicken, IMO isolating her will not solve the problem, and by introducing another bird is likely to cause stress to both birds and if your four are used to each other I would stick with them for the time being. Stress can induce even worse problems!
She has a problem with her digestion, which is a difficult one as her previous course of Baytril may have caused a reoccurance of her problem by eliminating some of her good bacteria which lives naturally in her gut
Some options would be firstly, do you hens have access to poultry grit? (oyster shell is different) having grit will help the grinding and motility of food in her digestive system.
Feed: Only give layers pellets,NO treats, not even mixed corn(harder to digest) for at least 2 weeks and make sure the feed is absolutely dry, for the time being keep the feeder in the house if you can. Add ACV to the water, for now at a higher rate 50ml per litre. Add a crushed garlic clove as well.
Find a good natural probiotic Avipro Plus is a good one that Aunty recommends. If the chook is on antibiotics still do not empty the crop manually but finish the course, this will clear any bacterial infection lurking. Keep on the probiotic, garlic and ACV. After the course ask your vet to prescribe Nystatin if your hen is still having symptoms, or maybe as a prophalytic due to her history.
The run: A deep layer of soft wood chips would be a good idea, maybe sink a tub of compost mixed with grit for her to dig around in she will be kept entertained and you can get sterile compost but ordinary compost should be fine!
Finally, personally I dont like the house in the link, I recognize the cheap wood, prone to warping and therefore damp, this will potentially give you all sorts of problems, plus that is a very small run.
Good luck!