I had a discussion with RJ about this some time ago and it ends with 4 space indentation.
Recently we have discussed line brakes in general and according to this discussion it has to be 8 spaces.
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Constructor_Initializer_Lists
(we double google's spaces)
For me both is fine, but I prefer 8.