3 season is all you need unless your looking at doing some serious winter camping in the mountains. Seriously I've slept in my 3 season Big Agnes SL1 in some nasty weather around the world and had no complaints.
With regards to somebody's comment above about using a 3 season tent for winter camping and ventilation. I find that I get way too much (frozen) condensation if I don't leave a one side of the rain fly out during the night. I usually leave the side opposite of the wind pulled out for the tent to breath better. Better not to have water frozen inside the tent come morning when the sun hits it or it will melt the ice that builds up and drips all over your sleeping bag and such. Difficult to dry things out in lower temperatures