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The Fourth Age of Kahless was a time period within Klingon history. The old Klingon calender system reverts to this time setting in the year 2400.
In the alternate timeline known as the War of the Prophets, it marked the occasion when the blue wormhole would merge with the red wormhole and wipe out all existence by the creation of a warp 10 shockwave. (DS9 - Millennium novel: The War of the Prophets)