How deep is your wood chip mulch. If it's 3 to 4 inches you needn't cover too much. If it's only covering the dirt then You will need to cover. If you have dry fallen leaves they make excellent winter mulch. They will get wet, but if you put them on dry there is little air pockets that keep the roots from freezing as hard, much like the snow does later on. Put a few around the plants now, but not too deep yet. You don't want mice and varmits making a nest and chewing on your plants all winter. After the ground is frozen solid them heap the leaves up but leave some of the plant showing.

Don't be too eager to remove it all in the spring either. I have lost more plants to hard spring or fall frosts than I ever lost with a hard winter. When The weather warms and the plants start to grow remove it from on top of the plant but leave it around the roots. A good thing to do with those zone 4's by the house os to build a wind shelter for them, A few stakes out from the foundation, around the plant and stapled with burlap or such will do.

Thanks for the quick reply oilpainter. My mulch is only really for moisture control, so I will be picking up a straw bale after work today.

I've heard that leaves as mulch can be bad for plants here because of our frequent spring (and sometimes fall) freeze-thaw cycles (it does at least soft freeze every month of the year here typically, as you might be used to also). Our fall and spring is when we receive the most precipitation, so the leaves just tend to rot and smother the crown of the plant.

Today being a very acute exception, we almost never have any substantial wind here. Too many trees and hills, I guess, but the wind shelter is interesting. So now - to mulch in our twice a year 30 mph sustained winds!