4? Only as a backup. If you're serious about heating with wood (the in-laws do) you're looking at 10 cords of hard maple, ash, and birch.
And some of us just leave the snow tires on and drive to Georgia on them. The math actually makes a certain amount of sense. On average, one set of winter tires , driven only during the winter months, will do a car for life. Say 10 years. You'll need two sets of 4 season tires during that period. So total up three sets of tires for a ten year car life. In that time, changing twice a year at an average of $10 per tire costs $80 annually. I pay $110 each for my winter tires, $80 for each 4 season. Total cost over 10 years is $1880. By just running with the winter tires all year round, I'll wear them down to useless in about 4 years on average. So I should scrap the car with 2 years left on it's third set of winters, needing two tire changes, totalling $1400. There's an actual slight decrease in the fuel milage, but then, most people don't keep their tires at the proper pressure and they wind up paying that anyways.