I don't think I would bother with cutting it back this year Cohouser since being in zone 4 it won't be long before you have your first killing frost and the clematis will be killed back to ground level anyways. If you have a fungicide readily available, it wouldn't hurt to spray any new foliage that might come out to protect it, but fungicides are preventative measures and will not get rid of existing mildew already on the plant. At least you now know that Peveril Pristine is susceptible to PM in your garden and you can start spraying it proactively next spring when it reemerges from its dormancy.

Now my new Entel has got it as well... I haven't been able to find any info on what kind of damage mildew does to the plant. Is it fatal if not controlled? Some of the Peveril Pristine has died, so I removed that part. It's also possible that I overwatered, which I guess could explain both the mildew and the death.

In my garden anyway, mildew won't kill a plant. It makes the leaves look like hell and they eventually shrivel up but the plant puts out more growth and the plant rebounds. The only bad thing is if the plant gets mildew, puts out new growth and then new growth continuously gets mildew. This will eventually over a longer time deplete the plant's reserves and it may die from being unable to photosynthesize--especially a newer plant that doesn't have great reserves stored in its roots and crown. Don't worry if mildew is an issue now as the plants should be going dormant in your zone anyways.

Now if you have overwatered and that is what is causing the dieback, then discontinue the watering and hope for the best. Root rot is another matter!