The Fourth Age was the age of the Dominion of Men.

It began, in the reckoning of Gondor, on March the 25th of the year Third Age 3021.

In the Shire Reckoning, it began with the passing of the Three Elven Rings and various heroes of the Third Age, after the defeat of Sauron, on September the 30th of the year Third Age 1421.

In the Fourth Age many of the Elves passed over Sea, and those of the non-Mannish races that remained in Middle-earth dwindled and hid, for their time was past and Men no longer understood them.

The Fourth Age was also called the New Age and the Younger Days.
Encyclopedia entry originally written by tarkena