What type and size of trees are you talking about?

I often overwinter trees in containers outside but I placed them in a sheltered area or sometimes I plant the pot in the ground and make sure they don't go to sleep for the winter with dry soil. I've had luck with several varieties doing this but I did it mainly because I wanted the trees to get some more growth on them before I planted them in their permanent spot.

I have a few shrubs in pots, one is a potted emerald/smargd cedar that I keep on the deck. For winter I threw the pot into my unheated shed and it seemed to overwinter fine. Else, my usual method is to sink the pots into the ground for winter up to the rim. As a precaution I may mound leaves or peat moss over this, and I try to choose a dry area such as near the house foundation (but not south side).

As for varieties, It's up to you. Some people say that if the plants are growing in pots above ground and staying outside all year, try plants that are rated for two zones colder than yours. There is no hard and fast rule, however. If you sink them into the ground, there's no reason why you can't grow zone 4 plants in pots.

Glen