Beekeepers seem to do well even if their hives have snow piled up high. They naturally stop rearing up young, huddle up in the middle of the hive, create heat shaking similar to the way humans do when cold. The honey and pollen they have stored gets them through the winter but you might supplement them by feeding them extra. It is probably the worst part of the year for them and is when a lot of hives are lost.

Four months is a long time but if you see bees around during the summer then that indicates they are surviving somewhere within a couple of kilometers.

An option might be to move your hive away to a warmer area during the winter so they would not have it so harsh.

Your question brings up something most people don't learn about until they start ordering live bees; there are many breeds of bees and several are popular for certain characteristics. For colder climates one might go with a Russian queen who is suited to long cold periods. The trade off is these bees may be more aggressive than say an Italian bee. Not that people don't do well with them or get stung so much more than others. And hybrid queens are also available so they have combined traits from different lines.

It would be very important for you to find other people with long winters and see if they are doing anything more specialized during the winter. It might mean having their entrance up higher, above the snow so they can still get out on the warmer days. Mike Bush is a notable bee keeper who lives with a lot of snow. http://www.bushfarms.com/beeswinter.htm