According to Osho, the 'four thousand years' remark isn't a scientific claim but a parable: the exact number is irrelevant. He uses it to show our life doesn’t end at the skin - we exist in interdependence with the sun - and to expose the mind’s obsession with trivial facts that make us miss the essential: awareness, impermanence, and living meaningfully now.
He just used a big number to show the sun keeps us alive and that arguing over numbers distracts us from paying attention to life right now.