Since I actually couldn’t tell you what specifically were the differences, I thought it was worth a read, and it turned out to be pretty informative. The short answer is that the four season tents have reinforced poles for higher winds and snow conditions, as well as more rounded roofs to allow snow to slide off. They also tend to have thicker walls for a warm escape from the winds, and other conditions.
The article also notes that they are generally too warm for any other conditions, other than the cold. That makes me wonder why they aren’t called 1-Season tents. Yeah, sometimes I think too much. 🙂