WordPress 4.0 is not a major update, at least referencing the scheme {major}.{minor}.{patch}. However, WordPress version numbering is explained here, and major versions are identified by the first two numbers, e.g. some recent major versions were 3.7, 3.8, 3.9, 4.0, and 4.1 coming in December 2014.
WordPress noted in the release post, “4.0 is just another number for us after 3.9 and before 4.1”. Treat this upgrade like you did when upgrading from 3.8 to 3.9.
I’m not sure if there will be a 4.0.1 minor release, usually referred to as Maintenance and/or Security releases. They usually come out a few weeks after a major release, and so far there has always been at least one released.