I have this problem sovled just now . prep. I set up my /etc/apache2/http.conf
LoadModule dav_module /usr/lib/apache2/mod_dav.so
LoadModule dav_svn_module /usr/lib/apache2/mod_dav_svn.so
LoadModule authz_svn_module /usr/lib/apache2/mod_authz_svn.so
It works for a period without 404 while I then try to browse to http://mydomain.com/svn/test_repo .But after I reboot my Linux, I get a 404 not found. Try to add the lines to /etc/apache2/conf.d.subversion.conf
LoadModule dav_module /usr/lib/apache2/mod_dav.so
LoadModule dav_svn_module /usr/lib/apache2/mod_dav_svn.so
LoadModule authz_svn_module /usr/lib/apache2/mod_authz_svn.so
My SVN works well now! I wish these recommendations can help you.