Synesthesia is a rare condition that causes come people to experience the sense of sight, hearing, smell, etc. differently than most people. For example, when they hear a sound, they don’t just hear it (the way most people do) but they might also experience the sensation of “seeing” a color associated with the sound. I am not sure that the author’s description of synesthesia is totally accurate because the idea of seeing a number 4 as blue is not something I have heard before in connection with synesthesia. If you want accurate information, you should research from a trustworthy source.