In a typical Linux and UNIX system run level 4 is not defined and run level 5 is graphical login. I don't know about Slackware so maybe they do use run level 4. I'm just saying it sounds weird to use run level 4.
~/.xinitrc is the old fashioned way to start applications automatically when X starts. It may still be the only method possible with the simple window managers such as twm, fvwm, etc. However if you are using a sophisticated desktop environment such as Gnome or KDE then they have their own autostart mechanisms. In KDE you can create a link in ~/.kde/Autostart to an application that you want to start when KDE login happens. For example if you wanted to start Xeyes automatically on KDE then the following commands in a terminal window (Konsole) would do it.
Code:
cd .kde/Autostart
ln -s /usr/bin/xeyes xeyes
I haven't use Gnome in a long time so I don't know about that. They probably have something similar to KDE.
Addendum: I see that David the H. posted three minutes prior to me while bgeddy and I must have posted within seconds of each other. Unfortunately all three posts are extremely similar. Extremely similar. That's weird. I promise that I did not see David's post before I started to write mine and I have to believe that bgeddy didn't see mine before he posted.
I suppose I'm making too much of this but the similarity with no useful additional information to speak between these posts of is really weird. Yeah. I'm freaking out. Freaking out!