The Joomlashack team is very engaged in testing RC4 and we're impressed by the many bugs that Joomla volunteers have squashed. While it's "close by," RC4 is still not 100% stable, and you shouldn't yet rely on it to deliver your critically important web content.
When will Joomla 1.5 move into the realm of "stable?" According to Joomla.org:
Joomla! 1.5 RC4 is by far the most stable release of version 1.5 to date. It has undergone a great deal of testing and bug fixing since RC3 and is considered very close to stable... Karibu marks an important milestone in 1.5's development, as barring any major issues we believe it will be the last release candidate, with the next release being the big one... Joomla! 1.5 stable. When that happens will in part be influenced by the Joomla! community.
So there you have it. If our Joomla community uses RC4 and tests it thoroughly, and no major problems are discovered, then the stable version of Joomla 1.5 will be coming in early 2008. Cool. Now go download and test it!