There are many ways to divide people into four personality types, from the classical—sanguine, choleric, melancholic, and phlegmatic—to contemporary systems such as the DISC profile. The Myers-Briggs system divides people into sixteen personality types. I just recently ran across the “enneagram,” an ancient  system for dividing people into nine categories.
There’s one thing advocates of all the aforementioned systems agree on: the number of basic personality types is a perfect square.
 Update: Maybe not so ancient. Promoters of the enneagram say it traces back to medieval or ancient times, but it looks like it traces back to only the 20th century, depending on how you look at it. The origins are strange and controversial. It’s the least reputable of the systems mentioned here.