the problem is that files created on an ext4 filesystem using extents. Extents are not supported by ext3. If you mount an ext4 as an ext3 filesystem, all already created files are still using extents.
To solve this, each file has to be recreated (e.g. by copying).
You can find more detailed discussions e.g. here:
If you are talking about the "root" or "boot" filesystems, I suggest to use fsarchiver to create an archive and to restore the archive with the mkfs=ext3 option.
Take a look at the quickstart guide.