From what I've Googled so far, there are lots of claims that the number 4 is considered unlucky in Chinese. 四 is understandably considered unlucky in Japanese and Korean because it sounds identical to the Sino-Xenic word for "death", 死 (in Japanese in particular, with the exact same accent pattern). In Chinese, this claim is a lot less convincing. Many articles, including these shoddily sourced ones from Wikipedia, Chinese numerology and Tetraphobia, stop short at claiming it "sounds similar to", "is nearly homophonous to" or "sounds a lot like" the word for "death". However, as this table displayed in the Tetraphobia article shows, 四 and 死 have different tones, except only in Shanghainese; even the supposed photo evidence is said to have been taken in an elevator in Shanghai. In most other dialects (see Wiktionary entries for 四 and 死), they always have different tones. In Mandarin, the syllable sì represents not just 四, but at least 53 other different characters, so if 四 is "unlucky" merely because it sounds "similar" to 死, what makes it especially different from those 53 characters? Or are they all "unlucky"? If we go back to Middle Chinese, 四 sounds identical to 肆 (which is, admittedly, used as an alternative to 四 in modern financial contexts), so is 肆 "unlucky" too?
So my question is, is 四 really "unlucky" because it sounds "similar" to 死? If so, why do only sǐ and sì sound "similar", but not sī for example? Is tetraphobia common in most dialects, or only in Shanghainese?