The four stages in the development of the Alter Rebbe’s Torah: דרכים very short thoughts that speak very passionately. The “ChaBaD” is very hidden here. as though they were simply to teach how to serve God with little “mind”. אגרות Longer thoughts that take on some ChaBaD (intellectual) form. [This idea is compared to the concept of a letter which is less permanent than a book, as explained in תורה אור לפורים). תורות Longer still, here there’s a real concept of understanding, and these are the basis of the מאמרים included in the תורה אור ולקוטי תורה. These are Torah [that didn’t need to be written down unless the listener would forget]. כתבים Longer still. Perhaps the use of the word כתבים here indicates the absolute need for them to be written.