actually runlevel determines a lot of things, one being starting X, but there are a lot more services that may be running in certain runlevels.
look at the folders in /etc/rc.d. In these folders you will see the links that start and stop services at the different runlevels, each runlevel has a folder. shutdown of your computer consists of bringing it down to runlevel 0, runlevel 6 is also a shutdown but instead of poweroff it is setup to reboot the system.
runlevel 4 is not normally used for anything special, however I think slackware used to run X in runlevel 4.
checkout these commands with man
chkconfig ( used setup links in the runlevel folders, determines what gets started and stopped)
service ( used to start a servie from the command line)
init ( or telinit, can be used to change runlevels from the command line)
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