There is no difference as far as the interpreter is concerned. Think of ; ;; ;;; and ;;;; as different heading levels.
Here is my personal use convention:
;;;; Top-of-file level comments, such as a description of the whole file/module/namespace
;;; Documentation for major code sections (i.e. groups of functions) within the file.
;; Documentation for single functions that extends beyond the doc string (e.g. an explanation of the algorithm within the function)
; In-line comments possibly on a single line, and possibly tailing a line of code