This morning, Dawn had a new small area of swelling on the bottom surface of her neck several inches below her chin, and by this afternoon it was larger, extending for about 5 or 6 inches down her neck. And several of the pleated areas on the side of her neck were hot and clearly sore to the touch. And although she didn't have a temperature, her demeanor was slightly depressed and she wasn't herself - she was quieter than normal and not as bright-eyed. I called the vet, and fortunately, our regular vet was on call and came very quickly. She agreed with me that swelling and heat shouldn't be returning after 4 days on antibiotics, and that it was clear that things weren't heading in the right direction.
So Dawn will be switching to a new injectable antibiotic tomorrow (details later), and tonight the vet, in addition to drawing blood to check her status, also sedated her and gave her DMSO through a nasogastric tube to help the inflammation and swelling. We've bumped her bute back up to 2 grams morning and evening, and I'll also be doing a DMSO sweat on the pleated and swollen parts of her neck. Once she's recovered from sedation, she can eat her normal rations and hay, and can go to turnout. The vet said the DMSO should make her feel a bit better by tomorrow.
I think she's going to be fine, but this cellulitis (or vasculitis - the vet said it could be either) is hard to get under control, and takes careful watching.