A 4-season tent is a tent suitable for winter use; for example, see this post. According to sectionhiker, the phrase is actually a misnomer:
Moreover the phrase “4 season tent” is a misnomer since it refers to winter tents, which you might not use the rest of the year because they could be too heavy or too hot.
This raises the question: when I'm hiking in the mountains, I might one day be camping in the valley with 10+°C (50 deg F)night time temperatures, and the next night at the mouth of a glacier with a high -5°C (23 deg F) (wind coming down). Are any tents suitable under both conditions at all?