The category is for articles and events specifically related to the decade of the 400s which begins in the year 400 and ends in the year 409.
Note that 400 is actually the last year of the 4th century; 401 is the actual first year of the 5th century.
Wikimedia Commons has media related to 400s
This category has the following 10 subcategories, out of 10 total.
Pages in category "400s"
The following 2 pages are in this category, out of 2 total.