First, you don't want to prune now. Pruning for a majority of deciduous trees and shrubs is done when they are fully dormant - without leaves - usually in late winter. This is the case with any kind of panicle hydrangea, like PeeGee's. Pruning too early (now) can still stimulate growth that can be damaged in winter. Since these types of hydrangeas bloom on growth that is produced the same season (new wood), there are no worries about losing flowers by pruning in late winter or early spring.

You can prune back by about a half to two thirds and to just before a leaf node. You could cut back further if you prefer but that tends to produce stem growth that is very limber and may not be any more supportive. It is best to leave some length of existing stem in place as this will have developed sufficiently strong wood to support new growth and heavy blooms.

I'd not worry about the lower leaves. Established paniculatas do tend to have somewhat 'bare legs' regardless but reducing the size of the plants should spark additional leaf growth next season that would have been shaded out by excessive topgrowth this season.

If the leaves are already turning and falling off, pruning couldn't hurt too much. Get a general shape now if you like and do the rest in early spring.

Pruning stimulates growth. If you prune before the plant is fully dormant, you do risk encouraging growth that can be more easily damaged by cold weather. The dieback does not necessarily limit itself to just the new stems but can be transmitted down into more mature tissue.

The leaves are off most of my PG's, and in zone 6.... To see some magical resurgence here let alone zone 4 would be amazing...

Thanks for the tips! Springwood, I'm surprised the leaves are off your PGs already. Mine are not only still on, but green. I'll wait til they fall to prune them - I kept myself busy over the weekend moving a bunch of raspberry volunteers and planting bulbs. :)