What does the 4th number mean in Java 9's version string scheme?

Question

According to this blog on Java 9\'s new version string scheme, the version is supposed to be like MAJOR.MINOR.SECURITY, i.e., there are supposed to be 3 numbers and

Answer 1

i.e., there are supposed to be 3 numbers and 2 periods in between.

Not necessarily and you can validate the versions using the JDK itself as detailed below.

In addition to the JEP which holds true as linked by @Stephen in the other answer, there has been an API addition to the JDK as well for the Runtime.Version which can be used to validate a given version string. This can be done using a sample stub as :

[I wonder using JShell could be interesting here, no IDEs!]

Runtime.Version version = Runtime.Version.parse("9");
version = Runtime.Version.parse("9.0.1");
version = Runtime.Version.parse("9.0.0.15");
version = Runtime.Version.parse("9.0.0.15+181");

The code makes use of the Version.parse that

Parses the given string as a valid version string containing a version number followed by pre-release and build information.

and can be further used(primarily) to get information like major, minor, pre-release and security number of the (runtime) version.