Ext4 mounting is supported with the filesystems-2.6.29.1-tinycore.tcz extension. You can add it inside the initrd (this is the procedure for current 2.9rc2, won't work with rc1):
# Assume initrd is extracted into /tmp/ex
mkdir -p /tmp/ex/opt/tce/optional
cp filesystems*.tcz /tmp/ex/opt/tce/optional
echo filesystems*.tcz > /tmp/ex/opt/tce/onboot.lst
Currently we don't have fsck / other ext4 tools, but we also don't recommend using ext4 in production with the .29 kernel.