While I got excited too seeing "mdadm 4.0" cross the wire, it's not a huge update but does have some useful improvements. It turns out the bumping of the major version number was done to reflect developer Jes Sorensen taking over maintainership of MDADM from Neil Brown.
But besides having a new maintainer, the mdadm 4.0 release does have fixes for IMSM RAID and clustered RAID. MDADM 4.0 also has support for IMSM 4k sector drives, failfast support, and other changes.
More details on mdadm 4.0 can be found via this release announcement.