Growing up hiking the southern Appalachians, I've never actually needed a four season tent. However we are starting to look at hikes around the country where the temperatures and conditions get worse in the winter. What's a good guideline for knowing when to bring the four season tent instead of the three?
I think there are two primary distinctions between "4 season" (winter-capable) and "3 season" tents: Air flow and room.
In a 3 season tent, you generally want good ventilation and you pretty much leave it at that. You have to consider this issue a lot more in a winter tent. The difference is that in a winter tent the wator vapor in your breath will condense and eventually freeze. This brings up a set of issues that just don't exist when ice can't magically form from nothing inside the tent. Winter tents can generally be shut pretty tight, but not completely tight. You need some outside air exchange, but it needs to be possible to limit and control this.
The other issue is one of space. I have a tiny 3 season tent that is just enough for me to lie down in. That's great because it's light and small and can be pitched in many more places that would be out of the question for a tent with a larger footprint. However, the downside of this is there is very little room inside. Most of the time, it's not that big a deal to have to open the tent to sit up. However in winter you really want to be able to do a few basic things inside the tent, like get fully dressed in winter gear.
Just out of curiosity to see what the issues really were, I took my little 3 season tent on a single overnight in the middle of winter. I knew there were going to be some un-comforts. The point was to understand them better. It was -5°F in the morning, so this was a decent test. Condensed and frozen breath wasn't that much of a issue because the tent is reasonably well ventillated even with all the rain covers in place. However, getting up and going in the morning was really awkward. When it's that cold you really don't want to get out of your sleeping bag to get dressed at all, and you really don't want to open the tent to do it. I managed it, but it took a lot of squirming around and significantly more time than if I could have sat up. Having a vestibule for boots that is accessible from inside would have been really nice.
On a separate note, I also learned that a single thermal foam pad under the sleeping bad when the snow is that cold isn't enough. The cold manages to get thru to you because the sleeping bag (was a nice down bag) is compressed where you are lying on it. Enough body warmth gets thru the compressed foam pad at pressure points that it re-forms the snow there a little and makes things quite more lumpy by morning than they were in the evening.
I am not really an expert in winter camping but I do live in the northern part of the Appalachians. You should base your decision on the following criteria:
Last but not least, you should pay attention to freezing rain. Freezing rain is much heavier than snow and at the same time, it does not slide as smoothly.
This being said, I was curious to compare my 3 season tents with a 4 seasons and I was impressed to see that based on some criteria, it could withstand more stress. I guess then that before considering buying a 4 seasons tents, you should check what your own tent could withstand.