As an addendum, it's pretty unlikely that the unified four nations are as old as the Avatar themselves, and that it's been exaggerated or conflated.
There can't be 53 Earth Monarchs in 10,000 years, for instance, that would make the average Earth monarch's reign be 188 years long. More likely it's only been for a few millennia that the Kingdom of Ba Sing Se conquered the whole continent and became the Earth Kings. We also know there's a long time period between Wan's time and the colonisation of the Southern Water Tribe, its reunion with the North, and the settlement of the Foggy Swamp tribe.
The Kyoshi novels also imply that the lifetime of Laghima and Shoken was "older than the Four Nations themselves". That was 4,000 years ago, and the first Avatar cycle began 10,000 years ago. The Avatar has clearly seen a lot of political changes.