There's nothing wrong (nor anything especially right) with the 5 post thing. The problem is in the 4 hour restriction.  Why? Here is my experience to date:

Let's say you've made your 5 posts... now you go on to a more interesting forum, and have something interesting to contribute, you go looking for the reply or quote button, but it's not there. What do you do? Say "screw it, that's right, I'm still a newb", then surf to another site.  You come back the next day, thinking, ok, i'm gonna put in my time.  You read a number of threads, maybe for 10 minutes or so, then you feel like you have something to contribute.  But still, no quote or reply buttons. Grumble, grumble, rinse repeat. Time goes up by only 10-20 minutes a day.

I completely understand the need for some newbie vetting process - I have moderated other forums, and know the challenges mods and admins face. But the 4 hour thing is chafing my you-know-whats.