As Ext4 was experimental during the release of Lenny, it wasn’t fully implemented. However, you can mount an Ext4 file system on Ext3 with Debian Lenny, but only in “Read Only” mode.
tune2fs -E test_fs /dev/XXX
mount -t ext4dev -o ro /dev/XXX /mnt/ZZZ
I had Ubuntu 10.04 Lucid Lynx and Windows 7 on two other partitions when I installed Lenny, I now have read access to Ubuntu’s Ext4 and read/write to Windows 7’s ntfs file system.