(defun swap-bytes (n)
  (rotatef (ldb (byte 8 0) n) (ldb (byte 8 24) n))
  (rotatef (ldb (byte 8 8) n) (ldb (byte 8 16) n))
  n)

(let ((n #xA1B2C3D4))
  (format t "~x~%" (swap-bytes n)))

--> D4C3B2A1
    nil