They haven't released any details on what this update does, but the file size for the update is about 10MB. That suggests that this is no more than a minor patch for the OS running on the Kindle 4 and not an update which will introduce a new feature.
I've installed the update and I don't see any changes.
Update: One reader pointed me to the explanation on Amazon.co.uk. This update adds a "Read for Free" feature for Waterstones, Amazon's UK partner. at least, the UK version of the update does this; it's not clear that the update I installed has the same option.