The expansion of the Sahara four thousand years ago forced what group to migrate across Africa, spreading iron?

Tempted by the Egyptians, but they weren't known for spreading across the continent and they didn't trust iron, they thought it was impure and ruled by the demon Seth. It seems more likely a sub-Saharan group so perhaps the Bantu people, but check your dates, as the Iron Age didn't really begin until 3200 years ago.