On Wed, 24 Sep 1997, Anthony Woo wrote:

I was reading a book on Tai Chi and I found that in both China and Japan
there are five elements--as opposed to the four European elements.  Notice
five inner Senshi and four outer Senshi.  Bit of investigation (and a long
streach of imagination) came up with this...
The five inners represent the five classic eastern elements.

Don't bother.  This is all in the Sailor Moon FAQ.