I've never heard of beekeepers burning the hives at winter.

The beekeepers I've seen all keep the hives sheltered over the winter. No-one I've known purposefully harms bees, they sweep them out of the way when removing the hive top and these slats from the inside. If they are stung, the bee does die, but suprisingly (it was to me at least) they don't get stung all that often.

Why would we all die in four years if there were no more bees? Seems like a very large leap there.