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We do know why, it's because death 死 and four 四 have the same pronunciation sǐ in Chinese (and shi in Japanese).
No shit, but why is it a homophone with such a common word?
homophones are common in Chinese and Japanese because there's only so many potential readings of a hieroglyph, but each one has a different meaning
Sure, but they're often different enough to to be obvious in context, or similar enough to have a shared etymology.
Tones came later in Chinese, so when you have 2 homophones with similar meaning and different tones, they're usually from words that had 2 suffixes, which were later dropped, but the tone of first part remained, 买 and 卖 didn't end up with the same word by coincidence.
Every language has homophones. Even before tones were created, sometimes there are just coincidences. As far as I know, there's nothing to suggest that the number four and death are inherently related in some way. No one is suggesting that knead and need are related even though they sound the same, and lead (the act of leading) and lead (the metal) may happen to share a spelling but they're still completely different words.