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What you are refering to is known as the Last Battle and the Day of Doom/Dagor Dagorath.

It goes something like this:

Long after the War of The Ring when all is at peace once more in the World it is said that Morgoth finds a way to break through the Door to the Night (Which is what he was thrown out of, it leads into the Void) and re-enter Arda.
He enters Valinor itself and with him he brings all the creatures of evil and corruption he has ever Possessed, including Maiar like Balrogs and probably Sauron.
This is the largest battle ever to arise and it takes place on the plains of Valinor. Ar-Pharazon who went crashing down into the abyss with the Isle of Numenor rises from the deep and fights for the valar i think.
It is said that during this Batlle Gandalf/Olorin slays Sauron and Turin Turumbar, a man slays Morgoth himself becuase of the Curse Morgoth put upon him and his family.
Morgoths, Sauron and the Balarog spirits then go back to Iluvatar for safe keeping so to speak.

To destroy Melkor you have to destroy the world itself (just like you have to destroy the Ring to vanquish Sauron). This is becuase Morgoth put so much of his power into Arda.

Once melkor is destroyed the world is broken and it is made anew. This is known as the second Great Theme becuase it does not have melkors discord in it like the first.

This was how the world was meant to be when the Ainur looked into the vision that Eru gave them before the world begun.

Some useful quotes:

"And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined." (Anulindale)
What Eru is saying here is that nothing can be done against his will becuase he that attemps this shall prove but his instrument in the second Great Theme.

"But I think it was not so. Manwë will not descend from the Mountain until Dagor Dagorath, and the coming of the End, when Melkor returns. 8 To the overthrow of Morgoth he sent his herald Eönwë. To the defeat of Sauron would he not then send some lesser (but mighty) spirit of the angelic people, one coëval and equal, doubtless, with Sauron in their beginnings, but not more? Olórin was his name. But of Olórin we shall never know more than he revealed in Gandalf." (Unfinished Tales)

"But Ar-Pharazôn the King and the mortal warriors that had set foot upon the land of Aman were buried under falling hills: there it is said that they lie imprisoned in the Caves of the Forgotten, until the Last Battle and the Day of Doom."(The Silmarillion)

It is doubtful that you will find any more quotes on this subject as Tolkien only had a brief idear of this. This however is the general idear.

If you are interested in hearing more about this and the Fourth age you should see this Fanfic from someone in the Plaza - http://www.lotrplaza.com/forum/display_topic_threads.asp?ForumID=21&TopicID=194270&PagePosition=1

It is not known when the Battle took place. More likely it was at the end of the fourth age.

After the Battle the Dwarves help Aule rebuild the world, but then along with the Elves and Ainur, they go back to Eru and the second great theme is devoted to the dominion of Men.

I don't know how you can say that TM. There were many long stretches when evil was not present and people could do as they likes.

You say life was dull, but each man could, if he wanted, get a horse and some provisions and become a wanderer like Gandalf. It did not have to be like the Middle-ages in 1100.

How could it? Admittedly there were rough times, but there were also many peaceful times.

As for the horses, the Druedain did not have them as they were secretive forest folk. Gondor did not have many of them becuase they just did not see the need for them as there stregth lied in the fortresses, as opposed to Rohan which need swift steeds to save the day. Not becuase Gondor was too poorto get them.