Oops, I thought that was katakana ム for a sec! Heh.
In addition to being the bopomofo equivalent of an "s", it's also the Japanese Katakana character for "mu".
I don't think it's such a big mistake since it's still the letter which represents the sound for both characters, anyway it's also strange
If the article had been about something dealing with the number four and the word death in Taiwan, then the use would be fine. But since 注音 (zhuyin, the proper name for "bopomofo") is not used in Shanghai, it makes very little sense. It would have made more sense to put "Si" in the image... though 四 or 死 would have been better.