The number four is "the number of form, of reason and of regularity". It quite fits Mizuno Ami because she is the voice of reason among the inner senshi, being the strategist and advisor of the group. She also represents order and regularity that starkly contrasts with some of the other senshis' whimsical and unpredictable attitudes.
People who are fours tend to be conservative (see? This is really you, Ami-chan! ^^), solid citizens. They are "essential to every community to prevent excesses, being the upholders of law and order, concerned with justice and equality". True to her number, it is usually Ami-chan who is the conscience of the group, who always reminds the others not to spy on Usagi and Mamoru when they kiss (see the SMR movie scene on the right. Of course, this is only one of those situtations!). ^^
Fours are dedicated to service, always accepting responsibilities and even creating them. Ami, being the four that she is, always offers to help people with their studying, and even got into some conflict with her principal for doing so. And also, Ami has patience, determination and energy to succeed in whatever she may do.
Ami, just like her fellow Fours, feel that they have to be honest and straightforward all the time, that sometimes in her being so, she is often taken advantage of by other people (this is where the "underdog" part comes in...).
She can be exceptionally resourceful when handling others (in fact, she's quite good at changing the subject before a full blown argument between any of them explodes! ^^), especially because she has a need for peace and order. Also, it is said that Ami-chan possesses more determination and strength of purpose than her friends know (This is why we shouldn't underestimate her too much, no?).
Ami-chan would probably make any lucky man a loyal and faithful wife someday--but she would have to work extra hard in making their relationship a sucess (as fours are not usually demonstrative of their feelings!).
So, is Mizuno Ami a perfect four?