The “4” in 4-season shelter comes from a reference that any model with this claim should be able to be used in “all 4-seasons”, though most are geared for winter conditions. Shelters that claim 4-season use are nearly always more robust than 3-season models and will be better in higher winds, more resistant to snow loading, and generally are warmer with less venting options.
While the name 4-season makes one think of winter, people tend to use them in more mountainous environments above treeline terrain, which can have “winter-like” conditions at any point of the year. People use 4-season tents when mountaineering, on multi-day ski tours, and on winter camping where their tent is likely to be exposed to wind, heavy rain, or snow — more so than while typically backpacking.
Three main types of 4-season tents
While there is no distinct line; 4-season tents group themselves into three areas.
Bivy Tents — as the name would imply, these models are geared towards shorter trips, with only a handful of nights spent in them on any given trip. Bivy tents prioritize low weight and a minimal packed size over comfort and sometimes stormworthiness, though many are made to be quite strong. Bivy tents nearly always have only two poles fashioned in an “X” shape (though in a few rare cases they do have three) as this is the most efficient use of materials and weight relative to interior space.
Bivy tents are almost always single wall, as single wall tents are lighter and more compact, and the reduced versatility of this construction is well worth the weight savings. Bivy tents tend to be less versatile, smaller, and less comfortable to hang out in.
Expedition Shelters — are best for trips in extreme alpine environments where strength, and to a lesser extent, comfort, supersedes weight and packed size in order of priority. These shelters are built for longer trips, so they tend to offer more spacious interiors and bigger vestibules for cooking and storing gear, and are characterized by their strong, albeit often heavier designs. More expedition focused tents have at least three poles but more commonly have four in the body and often a fifth to support a “hooped” vestibule. Buy a bivy tent for summertime alpine climbing trips or multi-day ski tours where you are likely to have to travel in a lot of technical terrains with your tent on your back or any trip where weight savings is paramount.
Double Wall Tents
Double-wall tents perform better in a broader range of conditions because there are two layers of fabric between you and elements. This creates a more comfortable space to live, with the most notable advantage coming in warmer, wetter conditions (handling internal condensation better). The inner tent provides condensation-limiting breathability while the outer tent supplies the weather protection. The inner tent is water repellant and breathable; it lets vapor pass through but prevents condensed water from falling on you (most of the time). Double-wall tents tend to be strong, but this is more a product of what they are designed for rather than two walls, actually making them stronger.