[alphapapa]

Wow! This is fantastic! This seems to do just what Parinfer does. Why isn't this mode more well-known?

This makes it even better:

    (defun my/lisp-dedent-adjust-parens ()
      (interactive)
      (save-excursion
        (x4-smarter-beginning-of-line)
        (call-interactively 'lisp-dedent-adjust-parens)))
    (defun my/lisp-indent-adjust-parens ()
      (interactive)
      (save-excursion
        (x4-smarter-beginning-of-line)
        (call-interactively 'lisp-indent-adjust-parens)))
    (local-set-key (kbd "<M-left>") 'my/lisp-dedent-adjust-parens)
    (local-set-key (kbd "<M-right>") 'my/lisp-indent-adjust-parens)

Now you can just M-left and M-right to adjust the indentation of any line, regardless of where the point is, and adjust-parens fixes the parens for you.