Printing with timer

[Heime]

I would like to insert the letter using a timer.  After each interval, the 
letter "T" is printed.
After four intervals, the characters are cleared, and the printing starts again.

Here X means whitespace

XXXX    ; After time interval t (all columns off)

TXXX    ; After time interval t (one column green)

TTXX    ; After time interval t

TTTX    ; After time interval t

TTTT    ; After time interval t

XXXX    

I have started with the following function.  Quite unsure if I should (and how)
the timer can be stopped.


(defun fire-indicator ()
  "TODO."
  (interactive)
  (insert "T")

(defun fire-clock ()
  "TODO."
  (interactive)

  (let* ( (tr (run-with-timer 3 1 (fire-indicator))) )
    ) )