Ext4 was not released as stable until kernel 2.6.28.
I think you need to mount as type "ext4dev" for earlier kernels.
You would also need the "test_fs" flag set:
tune2fs -E test_fs /dev/sda6
But an fsck will reset that flag, prevent this by creating the file
clear_test_fs_flag = false
You can manually clear the test_fs flag if needed:
tune2fs -E ^test_fs /dev/sda6