What happens when the sun scorches men with fire?
When the fourth vial is poured out, the sun scorches men with intense fire and heat, causing severe burns, yet instead of repenting, people blaspheme God's name rather than give Him glory.
Revelation 16 describes the fourth vial being poured out upon the sun, and power is given to it to scorch men with fire. The result is extreme heat that causes severe burns on the people of the earth. This scorching is connected to the earlier fourth trumpet judgment, in which the sun was smitten for one third of the day; since there is no record of the sun ever being restored to shine for a full day, the remaining hours of daylight become so intensely hot that men are scorched with fire.
Rather than turning to God in repentance, the people's response to this burning judgment is to blaspheme the name of God, who has power over these plagues. They refuse to repent or give Him glory, even while suffering under the heat.
This judgment is tied to earlier scriptures describing the earth being made empty, wasted, and turned upside down, with its inhabitants burned and only a few left, as well as the promise that the day of the Lord will burn like an oven, consuming the proud and wicked so thoroughly that they are left with neither root nor branch.
The scriptures
And the fourth angel poured out his vial upon the sun; and power was given unto him to scorch men with fire. And men were scorched with great heat, and blasphemed the name of God, which hath power over these plagues: and they repented not to give him glory.
Behold, the LORD maketh the earth empty, and maketh it waste, and turneth it upside down, and scattereth abroad the inhabitants thereof. And it shall be, as with the people, so with the priest; as with the servant, so with his master; as with the maid, so with her mistress; as with the buyer, so with the seller; as with the lender, so with the borrower; as with the taker of usury, so with the giver of usury to him. The land shall be utterly emptied, and utterly spoiled: for the LORD hath spoken this word. The earth mourneth and fadeth away, the world languisheth and fadeth away, the haughty people of the earth do languish. The earth also is defiled under the inhabitants thereof; because they have transgressed the laws, changed the ordinance, broken the everlasting covenant. Therefore hath the curse devoured the earth, and they that dwell therein are desolate: therefore the inhabitants of the earth are burned, and few men left.
For, 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.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.