Reply #4: I believe one current thought has it that less than four base pairs... [View All]

[TheMadMonk]

...is too rigid to allow for the necessary complexity to code for a viable living organism, and that more than four is too unstable to survive mutagenic processes for any great length of time.

These extras might well prove very useful in non-biological applications, but not to expand the lexicon of the book of life.

Four gives enough complexity to make things interesting to begin with and allows just enough change to ensure interesting things keep happening, while not permitting so much change that complex organisms are essentially impossible.