The Fourth Era is defined by the fallout of the kerymsept, and the rise of other races following the disunity of the Elves. This rise was both geographical, with many other races staking a claim in previously Elf-dominated lands, as well as intellectual. The Elves used to be the prime keepers of magical knowledge, but now, many other cultures made great advances, such as the Orcs of Karkarand.
The Elves themselves fortified what holdings they had left, such as the High Elves of Elleseand closing their borders to the outside world.
The exact length of the Fourth Era is hard to discern, due to the various methods of time-keeping used by different cultures. Generally, it is agreed to be somewhere between 1000 and 3000 years.
The Fourth Era came to an end with The Year of the Black Star - an event so notable, that all cultures have some record of it occurring.