Answer:
There is no "Linux 4" or "Linux 9." I'm not sure where people get these numbers from. They could be reading the version numbers of a particular distro (such as Red Hat Enterprise Linux 4 or SUSE Linux Enterprise Server 9). However, the Linux kernel is developed centrally and then subsequently used by all of the distribution vendors. The current stable version of the Linux kernel is 2.6.30; the 2.6 kernel line is expected to continue indefinitely.