Apple released its Mac OS X 10.5.4 update this afternoon, in what marks the fourth update to Leopard since it was released last October. The newest version of the operating system – available via the Software Update – contains the usual mix of bug fixes and security updates, addressing issues in AirPort, iCal, Safari, Spaces, Exposé, and just the general OS.

Besides fixing a number of major bugs identified since the release of Mac OS X 10.5.3, the update supposedly paves the way for the future replacement of its .Mac service, MobileMe, which was unveiled by Apple at its Worldwide Developers Conference earlier this month. Among the more general problems addressed in this update is an Adobe CS3 corruption bug with remote file saves and a chronic shutdown flaw. Also, AirPort appears to have received a tweak that improves reliability on 5GHz 802.11a and 802.11n networks. Check out the full details here.