Differences Between 3 & 4 Season Tents

Four season tents do not have mesh panels, they have mesh doors that can be sealed-off by canopy doors. That means extra zipper tracks, extra fabric, and therefore extra weight. Three season tents have exposed mesh panels that cannot be sealed-off. If you take a 3 season tent out on a cold winter day, you'll get pretty chilly.
Most four season tents have wider pole diameters, these poles are heavier, but stronger. Also, four season tents have poled vestibules, because we expect you to have to spend time inside your vestibule when the storms are raging outside.