I would probably use a layer of bubble wrap, either lining the barrel except for the bottom before planting or wrapped around the outside of the pot in winter. And I would use mulch on the surface. This will help reduce freeze-thaw cycles which tends to be one of the bigger problems for plants in containers in colder zones. You will want to read up on soils for containers, and know that you may need to change out the soil periodically. I would choose one that is small enough to remain within bounds (no sweet autumn clem or Betty Corning) and choose one that is hardy to zone 4 to increase the likelihood of survival.