500s BC · 400s BC · 300s BC
Decades of the 400s BC:
490s BC · 480s BC · 470s BC · 460s BC · 450s BC · 440s BC · 430s BC · 420s BC · 410s BC · 409–400 BC

400s BC may refer to:

• The period from 499 BC to 400 BC, almost synonymous with the 5th century BC
• 409–400 BC, rarely

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