The Mac updater is correct.
10.5.8 is the last available free update for OSX Leopard. http://en.wikipedia.org/wiki/Mac_OS_X_Leopard#Release_history
You are now faced with paying for an upgrade to OSX Mountain Lion (requires an upgrade to Snow Leopard first) but it's possible that your Mac may be too old to run it.
https://www.apple.com/au/osx/how-to-upgrade/
If that's the case then you'll need a new Mac to run LR4.4.