User blog comment:BlackWizardZeref/Fairy Tail: Chapter 436 Memoirs Review with a brief mentioning of Chapter 435 Victory Shout!/@comment-4059819-20150519074515/@comment-24546844-20150519193400

I thinkk 400 years is just a way to reffer to something that happened in that century. You don't need to be specific when talking about something that happened a long time ago. Like it could have happened around 350~450 and yet 400 could be used as reference.