Oh Jenny, what a bummer! I'm so sorry. Didn't manage to get on the laptop at all yesterday so unable to wish you goot luck at the vets.
Firstly did your vet say why the infection or where it came from? You didn't do an ultra sound or any other proceedure that was slightly 'invasive' did you - mares dont simply develop internal infections without good cause, they are too well protected against them.
IMO Saffire is a lot closer to foaling than 4 weeks anyway - I would say nearer to 10 days or there abouts, so her foal is pretty well ready to meet the outside world.
Detatching of the placenta usually means that you are about to have a red bag birth (my limited understanding of things), the red bags that I have had over the years have mostly been with mares going near or full term with their pregnancy, and you just have to be right there to break the bag and get the foal out immediately as once the placenta splits away completely the foal is no longer receiving oxygen. None of my mares had an infection and they all went on to have normal births later. But in all cases the breaking away of the placenta triggered labour very quickly and we were able to save all the babies - except the first as we had not heard of this and waited for the vet to arrive resulting of course in a dead foal.
Perhaps your vet is hoping the anti-b's will keep the placenta attached for a bit longer and hopefully this will mean that the foal is still able to get oxygen and nourishment for a bit longer? But watch Saffire like a hawk (I know you will) and if she goes into labour I would call your vet immediately to get her to come, and then concentrate on getting that foal OUT as fast as you can - there is a very good chance that he/she will be born alive!
Hoping that Dr Taylor might answer your post on the main forum, and wishing you, Saffire and her little baby the very best of good luck.
What was the news on Dusty?
Big ((((((((((HUGS))))))))))