That’s the approximate number of dances Schubert wrote throughout his life. Together with his songs these pieces contain some of his most idiomatic music. They are especially noteworthy in their relationship to Austrian folk music combined with an endless melodic variety and subtle, often surprising harmonics.
When I first came across these works it felt that he had poured out these musical ideas, as he was aware there wouldn’t be enough time to work them into full works. So those 400 dances seemed to me like a huge library ( or stoneyard ) of unused musical ideas, which would have had the thematic potential to be turned into hundreds of sonatas and symphonies.
Anton Webern was a great admirer of Schubert’s dances and orchestrated 6 of them.
Here are some examples plus the Webern version.

This last set Webern orchestrated in 1928, and conducted it in 1932. Here is the rare recording.