There are some "human brain" oriented arguments for using
consistent indentation as well, and -- depending on font --
those arguments imply that 2-space indentation is often NOT
sufficient (but 4-space indentation is).  The trick is that
strong lines must be nearly horizontal or nearly vertical
to trigger special pattern-recognization cells in the
visual cortex.

I am not sure that anyone knows *why* this is yet; the theories
I heard and lean towards myself are that they are for recognizing
the horizon line and things like tree trunks.  Since those cells
*are* there and are doing work all the time, whether or not you
want them to, you might as well put them to good use. :-)

(2-space indents are insufficient when using "thin" fonts and
sufficient when using "fat" fonts -- you need to get indented
text to match up vertically more strongly than `over 45 degree
angle' bogus matches and the like.)

Chris