Because the Japanese numbers 4 and 2 are pronounced "shi" and "ni", respectively. These words are homynyms for (they sound exactly like ) "death" and "you" in Japanese, although they obviously are represented by different kanji. For this reason, Japanese has an alternative pronunciation for 四 - "yon" - to avoid saying "shi" (死).

Here's an interesting mathematical curiosity regarding Japan and the number 42. It draws together important dates in Japanese and American history. - February 10th, July 4th, September 2nd, Dec 7th and only these dates. February 10th is the eve of Japan's Foundation Day which is often celebrated with a night-time street parade. September 2nd is the day the war officially ended with the signing in Tokyo Bay of the terms of surrender. December 7th was the day the Pacific War began with the attack on Pearl Harbor. And July 4th, the birthday of the victor and today an occasion for fireworks display in both countries. Now for the breathtaking mathematical fact surrounding these dates. The dates correspond to the 41st, 185th, 245th and 341st day of the year in a non-leap year year. If you calculate the sum of the two nearest factors of each number, (making sure that the pair of factors must be equal to the number when multiplied) you will find the sum of each pair will be 42. The factor pair are (41,1) (37,5) (35, 7) and (31,11). This only holds true for these four days of the year. 42 was made popular by Douglas Adam's A Hitchhiker's Guide to the Galaxy. More recently a game called Squaring 42 led to the discovery of the Holy Grail Spiral of Life. (935-QKK) As John mentioned, there is an association with death regarding this number - but one can also find in it strong connections to Life and Peace. 42 is an important number to remember to keep the peace of our nations.