I followed the Ubuntu wiki instructions on migrating from ext3 to ext4. After the tune2fs bit I rebooted as instructed and the system could not mount the new filesystem. Errors about bad superblocks and bad group descriptors, but nothing being done about them. fsck did not run correctly and I was left with an un-mountable drive, so I could even mount as ext3. Ahhhh....
Fortunately I have a live CD install of 9.10 on USB from which I could boot and run a fsck.ext4 on my partition.
So my suggestion is that before migrating from ext3 to ext4 do two things:
- BACKUP all your data
- Do the migration from a live CD / USB. The advantage of this is that you save on all the reboots too.