The tool shows the partition as ext3/ext4 because, although ext4 is an improvement over ext3 (increased max file size, increased filesystem size, etc.) they are fully compatible and you can mount an ext3 filesystem as ext4. So it makes sense to show it as ext3/ext4.
The tool also displays "Contents: ...(some contents unreadable)" because, as @UlrichSchwarz wrote in the comments, you ran it as an unprivileged user, so it isn't able to count all the files to which your user has no access: other users' homedirs, some system directories which allow only access to the root user, etc.