Since I started with Slackware it has always been 6 VTs in runlevel 3, only one in runlevel 4 (console 6). That is the expected behaviour. The X server always runs on 7 and above and no one has suggested differently. Only consoles with a '4' in their definition line will start in runlevel 4. Your posted file (the default) only has '4' in c6, and hence you can access console 6 normally -- but none of the other consoles are available. X is still on 7 whether in runlevel 3 or 4. If you achieved the behaviour in your last post on a previous Slackware version, it was either configured strangely (ie not the default) or you somehow corrupted your installation (or it was buggy but I very much doubt that). What you are seeing matches the expected behaviour *exactly*. To get all consoles available in runlevel 4 just add 4 to the list of options for c1-c5.
#These are the standard console login getties in multiuser mode:
c1:12345:respawn:/sbin/agetty 38400 tty1 linux
c2:12345:respawn:/sbin/agetty 38400 tty2 linux
c3:12345:respawn:/sbin/agetty 38400 tty3 linux
c4:12345:respawn:/sbin/agetty 38400 tty4 linux
c5:12345:respawn:/sbin/agetty 38400 tty5 linux
c6:12345:respawn:/sbin/agetty 38400 tty6 linux
Seems to be what you're looking for. You can try to convince us that you're correct...but you aren't, and that inittab matches up exactly with what should be happening.