It's true, they tend to avoid it because 四 (si4, "four") sounds similar to 死 (si3, "die"). I don't know why they settled on that way. It seems pretty arbitrary,

It's true, they tend to avoid it because 四 (si4, "four") sounds similar to 死 (si3, "die"). I don't know why they settled on that way. It seems pretty arbitrary, though. I'd like to find out more.