The fruit is produced on last years new growth. In zone 4 the bush will stop growing about this time or often sooner. The new wood at the tips of this yrs growth will form fruit buds for next yrs fruit.

So, if I do prune them, then best time is shortly after berry harvest.

Should I presume that pruning will help the next years harvest?
Anything special about pruning other than time of year?

Well usually canes 5 years or older are removed as they become unproductive. At least what I've read. I myself want to see a decrease in production before I remove them. I have some plants approaching 4 years.
I usually remove weak canes as berries drop to the ground. Canes crossing one another, dead, etc.

I prune Northern Highbush regularly when they turn about five years old and do it in the late Winter to early Spring.
The plants should be a mix of a few older canes and some newer canes after they reach that age. Brady

Oregon State University has a nice video guide to pruning high bush blueberries.

Here is a link that might be useful: Guide to Pruning Blueberries

"Here is a link that might be useful: Guide to Pruning Blueberries"

Excellent video. Actually, I never thought of pruning them until this year and figured it only to make it easier to move it.

I have two bushes that are over 40 years old, one that is about 5 years and one new one purchased this year (Blueberry Kabluey).

I am really enjoying the adventure to get these producing again.