The Fourth Vial isn't a repeat of the Fourth Trumpet.

        Here the heat of the Sun is much greater and so great that men are scorched by it.  It is the time spoken of by Malachi, "Behold, the day cometh that shall burn as an oven; and all the proud, yea, and all that do wickedly shall be stubble; and the day that cometh shall burn them up, saith the Lord of Hosts, that it shall leave them neither root nor branch"; and the time is located just before the Sun of Rightousness shall arise with healing in His wings.