You are right... some distros do use run level 4 for xdm/gdm/kdm (IOW full graphical interface) such a distro is SLACKWARE. This is a Slackware based thread...
Taken right from the inittab file on a slackware box...
# These are the default runlevels in Slackware:
# 0 = halt
# 1 = single user mode
# 2 = unused (but configured the same as runlevel 3)
# 3 = multiuser mode (default Slackware runlevel)
# 4 = X11 with KDM/GDM/XDM (session managers)
# 5 = unused (but configured the same as runlevel 3)
# 6 = reboot