With Cubism 4 Editor, the data created with Cubism 3 Editor can continue to be read and used.
There are no changes to the file extensions.

However, model files and animation files created or saved with a newer version of the editor cannot be opened correctly using an older version of the editor, and there is the risk that the files may be damaged.

If you attempt to use an older version of the editor to edit data created with a newer version of the editor, you must accept responsibility for all risks involved.

For the file extensions, please check the following manual page.
Live2D Cubism Editor Manual / File types and extensions