The beginning of the 4th age is marked by the arrival of Elves in Thaeria. Though many civilisations formed from their own advancements it was a time of great expansion and technological change and the beginnings of the racial integration which is so normal in our modern world. It's exact length is unknown, for records are spotty, and though historians can often track millennia of events forming parts of the timeline, it's not always easy to say how these parts go together, or that they didn't just run alongside each other. Current estimates say it must have been at least 5000 years long, but was probably nearer to 12000 years.