If you were to rebuild 4chan today...

...what type of data storage layout would you use?

Would you remain with a mostly flat file layout and what technologies would you use to implement such a layout?

Personally I'd go with JSON files and some random JSON library for writing, cache and reading stuff.

Don't need a persistent database at all. Just use JSON with redis. Think about it, image boards prune content very rapidly so there's little use for a database. Archive 404'd threads as serialized JSON or plain HTML.