400 was a number.
According to Maaga, she and her Drahvins had crashlanded on a deserted planet along with the Rills four hundred "dawns" earlier. (TV: Galaxy 4)
Hearing the Eleventh Doctor give his age as "twelve hundred and something", the War Doctor noted that that would make him 400 years older than himself. (TV: The Day of the Doctor)
In a linear sense, the Last Great Time War lasted for at least four hundred years. (PROSE: Engines of War)
Arcadia was defended by four hundred sky trenches. The Daleks broke through them all in the last day of the Time War. (TV: The Last Day)