Emacs Shift-Tab to left shift the block

I made some functions for tabbing a region over by four spaces left or right depending on if you use tab or shift+tab:

(defun indent-region-custom(numSpaces)
    (progn 
        ; default to start and end of current line
        (setq regionStart (line-beginning-position))
        (setq regionEnd (line-end-position))
        
        ; if there's a selection, use that instead of the current line
        (when (use-region-p)
            (setq regionStart (region-beginning))
            (setq regionEnd (region-end))
        )
        
        (save-excursion ; restore the position afterwards            
            (goto-char regionStart) ; go to the start of region
            (setq start (line-beginning-position)) ; save the start of the line
            (goto-char regionEnd) ; go to the end of region
            (setq end (line-end-position)) ; save the end of the line
            
            (indent-rigidly start end numSpaces) ; indent between start and end
            (setq deactivate-mark nil) ; restore the selected region
        )
    )
)

(defun untab-region (N)
    (interactive "p")
    (indent-region-custom -4)
)

(defun tab-region (N)
    (interactive "p")
    (if (active-minibuffer-window)
        (minibuffer-complete)    ; tab is pressed in minibuffer window -> do completion
    ; else
    (if (string= (buffer-name) "*shell*")
        (comint-dynamic-complete) ; in a shell, use tab completion
    ; else
    (if (use-region-p)    ; tab is pressed is any other buffer -> execute with space insertion
        (indent-region-custom 4) ; region was selected, call indent-region-custom
        (insert "    ") ; else insert four spaces as expected
    )))
)

(global-set-key (kbd "<backtab>") 'untab-region)
(global-set-key (kbd "<tab>") 'tab-region)

Edit: Added Maven's code to check if in minibuffer, as well as for tab completion in specific buffers (shell, for example).

To do that, I use the command indent-rigidly, bound to C-x TAB. Give it the argument -4 to move the selected region to the left by four spaces: C-u -4 C-x TAB.

http://www.gnu.org/s/emacs/manual/html_node/elisp/Region-Indent.html

This removes 4 spaces from the front of the current line (provided the spaces exist).

(global-set-key (kbd "<S-tab>") 'un-indent-by-removing-4-spaces)
(defun un-indent-by-removing-4-spaces ()
  "remove 4 spaces from beginning of of line"
  (interactive)
  (save-excursion
    (save-match-data
      (beginning-of-line)
      ;; get rid of tabs at beginning of line
      (when (looking-at "^\\s-+")
        (untabify (match-beginning 0) (match-end 0)))
      (when (looking-at "^    ")
        (replace-match "")))))

If your variable tab-width happens to be 4 (and that's what you want to "undo"), then you can replace the (looking-at "^ ") with something more general like (concat "^" (make-string tab-width ?\ )).

Also, the code will convert the tabs at the front of the line to spaces using untabify.