The root filesystem is mounted read only. This runlevel is commonly used when any other runlevel fails to properly boot the system. This runlevel is useful for troubleshooting network services, and maintaining shared filesystems. The system is in normal mode. On some systems such as Slackware, runlevel 4 is equivalent to runlevel 3 with graphical logon enabled.
All services running and graphical logon is enabled.
Changing Runlevels One of the key features of the init process, and perhaps the most confusing, is it's ability to move from one runlevel to another. To change to another runlevel, use the init command. This would stop most services, and bring the system into single user mode. Each runlevel has a directory of symbolic links pointing to corresponding scripts in the init.d directory. The names of the symbolic links found in these directories are significant.