4 Season Boots

Four season boots are made to be used in winter and to take crampons. But they can also be used in spring, summer and autumn, hence the name. 

At one time, all boots were made to be used during all four seasons, and walkers simply wore the same pair all year round. There's no reason why you can't do that with these, as the name is four season, not fourth season.

They might be a bit heavy on your feet, but the best of them are certainly comfortable enough for year round use. If your walking leads over rough rocky mountains, you'll be better off with a pair of these than many lighter three season boots, which will soon crumple under the strain. 

The best four season boots are expensive, but remember that the best of them will serve you throughout the year, so you'll only ever need to buy one pair. 

A winter boot must allow the wearer to kick steps in snow, traverse frozen slopes and wear crampons. So the stiffness of the sole is very important. To test stiffness, grab the boot at the toe and heel with your hands and try to bend it. The more suitable the boot is for winter, the more difficult it will be to bend.

You can also try and twist the boot between toe and heel. Again, a boot that is flexible will give you bruised toes when you kick steps, and will roll off a ledge when traversing a snow slope. 
The uppers of the boot need to be pretty stiff to withstand winter use too. So ideally they are made from leather that is around 3mm thick. A stiff upper provides better support and protection to the foot, which you'll need if you're going to start kicking lumps of ice off a snow slope to get a firm grip underfoot. A thick upper will also keep your feet dry for longer, as it will impede the flow of wet snow and water.

Most important of all perhaps is a good sole. If you want to stay upright on snow and all that thick mud that collects in the valleys during winter, you'll need a boot with a deep tread. Many modern soles are also designed with anti-clog sole patterns, so they shed snow and mud more easily.
Fit and comfort are important, so always try a number of boots to get the best fit. Just because someone recommends a boot, it doesn't mean that it will fit your shape of foot.

One of the biggest debates that arises when you mention four season boots is their suitability for crampons. A crampon will stay on a boot better if the boot is totally stiff, but for more comfort when walking many crampons will also fit slightly more flexible boots without any problems.
A number of manufacturers are now starting to grade their boots and crampons so that a level of compatibility can be achieved. The system used is outlined below and it follows that boots in the B3 category are ideal for C3 crampons, but will also take C2 and C1 crampons, while a B1 boot can only be recommended for use with a C1 crampon.

B0 (Walking boots) Not recommended for use in snow and ice conditions. These boots are unsuitable for crampons, because they do not have sufficient lateral or longitudinal rigidity, and their uppers are too soft to withstand the pressure of crampon straps.  
B1 (Hillwalking boots) Provide sufficient performance to be used in winter hill conditions with caution. Flexible walkers' crampons with strap attachment (Crampon grade C1) can be used for limited periods. The boots have a reasonably stiff flex, and the uppers and ankle provide enough support for traversing relatively steep slopes. 
B2 (Mountaineering boots) Provide sufficient performance to be used for winter walking, mountaineering and glacier traverses. They can be used all day with  articulated crampons which have straps or a combination of a strapped front and clip-on heel (Crampon grade C2). The boots have a stiff flex and a supportive upper made from leather which is around 3mm thick. 
B3 (Climbing and mountaineering boots) Provide sufficient performance to be used for winter mountaineering and ice climbing. They are compatible with articulated or rigid crampons with straps or clip-on systems (Crampon grade C3). These boots have a fully stiffened sole and many have a stiff upper that is made from plastic.  

C1 Flexible crampons attached with straps, with or without front points 
C2 Articulated crampons with front points, attached with straps or straps at front (ideally a French ring system) and a heel clip. 
C3 Articulated climbing or fully rigid crampons attached by clip-on system of toe bar and heel clip.