Open Question: What weather requires a 4 season tent?

I'm going travelling and want to go as light as possible, so if I don't need a 4 season tent I can cut back on weight with a 3 season.

I don't really know what a 4 season is actually aimed at beyond the 3 season. Is it protection against very heavy winter, snow, etc. or will I need it even for lighter winters, or would a 3 suffice?

Source:
http://answers.yahoo.com/question/index?qid=20110805105701AAfRKIM

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