The last of the four ages chronicled by Tolkien, and the one about which least is known (including its length). The Fourth Age was held to have begun with the passing of the Ring-bearers over the sea from Mithlond on 29 September3021 (Third Age), though in Gondor it was reckoned as beginning on 25 March of the same year (the second anniversary of the Downfall of Barad-dûr).
The History of the Fourth Age
Of the history of the Fourth Age we have little more than hints, and nothing at all of any substance after the second century of the Age. Most of what we know is restricted to the Shire and the Reunited Kingdom, which is natural as these two regions were the source of the histories of the Third and earlier Ages.
Across the wider lands, a peace descended, and though Elessar still at times rode against distant foes, for the people of the Two Kingdoms this was a time of prosperity and plenty. KingElessar himself gave up his life in IV 120, and was succeeded as High King by his son Eldarion. As time passed, the Shadow of Sauron became a distant memory, and strange cults and societies grew up in Gondor. These were the subject of Tolkien's abandoned sequel to The Lord of the Rings, entitled The New Shadow: the few pages of the story that he completed can be found in volume 12 of The History of Middle-earth.
The Fourth Age raises a question that tends not to apply to the preceding Ages, in that the change from the Third Age to the Fourth leaves us with three different, overlapping calendars. The New Reckoning reset the year number at the beginning of the Fourth Age, but the Shire-reckoning continued without a break. What's more, we occasionally see years of the Fourth Age expressed in terms of the Third.
It's therefore necessary to find a consistent way to convert between the three dating methods. There are several points in The Lord of the Rings where Tolkien gives us the same date using various systems, so in principle it should be easy to make the calculation. A problem arises, however, because Tolkien uses two different conversions in different parts of the book.
Method A: Simple Continuation
Perhaps the obvious method would be to continue the count of years directly from the end of the Third Age into the Fourth, so the sequence of years would run III 3020, III 3021, IV 1, IV 2, and so on. Using this system, we can convert a Third Age date into a Fourth Age date by simply subtracting 3,021, or to convert Shire years, we subtract 1,421.
This approach is supported by several cases in the Appendices to The Lord of the Rings:
A more involved approach is to consider the last year of the Third Age to also be the first of the Fourth. On this system, the count of years would proceed as III 3020, III 3021, IV 2, IV 3, and so on. In other words, both 'III 3021' and 'IV 1' refer to the same year. This idea is supported by another reference in Appendix D:
This is explicit that there was an overlap between the two systems, at least in official records. On this system, then, we would convert years of the Third Age to the Fourth by subtracting 3,020, and Shire years by subtracting 1,420.
Though this appears to be the 'official' calculation, there are actually fewer examples of it in practice to be found in The Lord of the Rings:
This confusion of systems is also seen in the draft texts of The Lord of the Rings published in The History of Middle-earth (especially volume 12). Method B is stated as the 'official' mode of calculation, but in the actual text Method A is used more commonly.
For the purposes of conversion on this site, we use Method A (which is not only more intuitive, but has rather more references supporting it). This means, unavoidably, that a few conversions are in conflict with statements in The Lord of the Rings (for example, IV 172 translates as Shire year 1593, despite the clear statement in the Prologue that it was 1592). To maintain consistent conversions, the occasional discrepancy like this is unfortunately unavoidable.
Although we have no records of the later Fourth, or any following Age, Tolkien makes a brief allusion to the future of Middle-earth in a letter written in 1958: "I imagine the gap [between the Fall of Barad-dûr and modern times] to be about 6000 years; that is we are now at the end of the Fifth Age, if the Ages were of about the same length asS[econd] A[ge]andT[hird] A[ge]. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh."
This note is especially interesting, as it gives some ground for bringing Tolkien's dating system up to date. The fact that we are 'at the end of the Sixth Age, or in the Seventh' hints strongly that Tolkien saw some important historical event as marking the recent (or imminent) end of the Sixth Age. Each of the three Ages we know about ended with a great war and the fall of a tyrant, and Tolkien was writing just thirteen years after the end of the Second World War: could there be a connection?
While this is circumstantial at best, it does seem to hint that perhaps the Downfall of the Third Reich was to the Sixth Age what the Downfall of Barad-dûr was to the Third. If so, we can 'reset' the calendar in 1945, which would be the first year of the Seventh Age. The letter quoted above, then, would have been written in VII 14, which explains Tolkien's reference to the change of Age, while the year 2000 would be VII 56.
It's important to stress that there's absolutely no direct evidence for any of this - it's just harmless speculation. We do know, though, that Tolkien was fastidious in calculating his dating systems: it's unlikely that he would have mentioned a change of Age if he didn't have solid reasons for doing so.