The 4 hours, especially, makes it even more of a pain in the a** for scammers.  Most scammers are going to be too lazy to go through the process of setting up an account, making 5 posts, and reading the forum for 4 hours.

You also overlook the point that the 5 posts / 4 hours rule also helps with the purpose of making new uses familiar with the forums.  This system cuts down on duplicate messages, since the new user has been able to read more posts on the forum.

As you said, scammers are still going to make accounts, but they won't be able to post on them.