Ext4 is forwardcompatible in that you can mount an ext3 file system as an ext4 file system.
ext4与ext3是向前兼容的，这样就可以将ext3文件系统挂载为ext4文件系统。
3
The substitution of the equivalent types depends on the compatibility of types, whether they are fully compatible, forwardcompatible, or backward compatible.