Maybe he means "The theory of relativity in words of four letters or less [than the theory of relativity]" -- i.e., it is an _incomplete_ explanation -- which some of the people on this page seem to agree with.
Seriously, I thought this was extremely clever, and the short, clipped style reminds me of Hemingway or Mamet.
You don't always use "fewer" when talking about discrete units. For example, you say "less than 5 minutes ago". The same goes for measurements of length (less than 3 meters). I think "less" works here because letters is a measurement of length.