The Fourth Age of Arda. It was known as the Age of Men, because thereafter mortals would consolidate their dominion of Middle-earth. These would be fading years for all other races. The ReunitedKingdom is a testament to this development.
In the first few centuries of the FourthAge, an age of peace and renewal descended upon Middle-earth which had not existed since the FirstAge (if at all). The Men of Rhûn and Harad made peace with the Dúnedain. Elves came to dwell in Ithilien; Dwarves came to Aglarond. Trees came back to live in the vale of Isengard. The Shire was declared off-limits to Men, but at the same time, Hobbits became more interested in and engaged with the wider world.
But many of the Elves of Middle-earth, especially the High Elves, took the opportunity in these years to pass into the West. And although many of the Orcs of Mordor were wiped out in the WotR, many more still lingered in deep caverns beneath the Misty Mountains, cowed for a time, but not defeated. The abandoned New Shadow hints at a renewal of Morgoth-worship during the reign of Eldarion, the son of Elessar.
An interesting "speculative history" of the Fourth Age is provided by Michael Martinez at http://www.suite101.com/article.cfm/4786/31389