What is the difference between GRASS 6.4.0 and GRASS 6.4.SVN?
6.4.0 is an official release, and 6.4.SVN tracks the branch in an SVN repository, and contains the latest changes to that branch. Because 6.4 is the current stable release, there should only be minimal differences between the two. Most users want to stick to official releases, but if you're digging deep into GRASS it may make sense to have a copy which is compiled directly from SVN, so you can try out changes as they're made by the developers.
Just guessing but I would say GRASS 6.4.0 is the stable version and GRASS 6.4.SVN refers to version in SVN source control, found here: http://trac.osgeo.org/grass/wiki/DownloadSource