Recently I was introduced to the 四字熟語【よじじゅくご】:
... to describe the type of changing weather one often experiences around late Winter/early Spring.
As to the literal meaning of this expression (and where it came from):
The 寒 and 温 in this expression are fairly obvious. For the 三 and 四, I automatically assumed a construction similar to the famous idiom: 七転び八起き【ななころびやおき】, where the "8" merely means "one more time than 7" and therefore something like "for every time you fall you get up, and then one".
With that perspective, 三寒四温 would be (very roughly) something like: "for every time it gets cold, it gets warm again". At any rate: some idea of a back-and-forth (ending on warm).
However, Wikipedia gives a completely different explanation:
In their definition, the "3" and "4" correspond to somewhat specific durations, and together make a "one-week cycle" of cold and warm characteristic of the season.
As much as I hate to question a Japanese Wikipedia page, this seems surprisingly specific and does not sound particularly grounded in any scientific or folk theory I have ever heard of. The reference links on the Wikipedia page are more illustrations than explanations and not helpful there.
Has anybody ever heard of this expression and its possible origins? Is there any substance to this "one-week cycle" of late Winter in older Japanese folklore?