It seems to be 4.1.

Given the mess we had with 3.x having about twenty different codenames, the point of the bora/chinook/diablo/elephanta was to clarify the codenames within each major release.

If Diablo is 4.1 after all, this could get confusing down the line; but matches with the comments in this thread from Texrat and Mara about the (relatively) contained set of changes.