Knoppix mirrors seem to have a new knoppix-dvd folder on the same level as the knoppix folder, so you won’t be able to find the download from the mirror site without going up one level. Here’s the changelog, straight from a mirror.
* V4.0DVD-2005-08-16
– Project split into DVD and CD edition
– OpenOffice 2.0 build 104 (english/german)
– Lots and lots of package updates
– Bugfixes: floppyconf, knoppix-installer
– Kernel 2.6.12(.4) update
– KDE 3.4.1 from Debian/experimental
– correct agpgart module detection
– tulip PCI id fix in hwdata
– ralink and madwifi driver updates
– hostap added
– removed “slow USB block device” (ub) option (problems with usb-storage reported)
– chmod cdrecord/cdrdao to no longer run suid root because of changed kernel ioctls
– Added development packages: eclipse, mono
– Added most kde-i18n langiages from unstable
– Added knoppix books and Open Source Jahrbuch
– Added m23 software distribution system
– Added Knoppix menu item for setting root password
– Added AVM modules for DSL and ISDN cards
– Added ISDN/DSL configuration packages from Kanotix
– Added alternative desktops: gnome, ratpoison, xfce4, openbox
– Added (too many to list) packages from Debian/sarge (abiword, gnumeric…)
– Replaced mozilla-suite by mozilla-firefox and mozilla-thunderbird
– linux-ntfs (sourceforge) CVS snapshot