Any ideas?

You don't, despite the similar name Safari for the Mac, Windows, and iOS are all different version of Safari that only work on their respective platforms. Even the iPhone and iPad versions are not identical, and can not be run on the other device.

The iOS version's full name is actually Mobile Safari. It comes preinstalled on the iPad and is only updated when the iPad receives an iOS update.

The Safari 4.3 you are looking at must be the Windows version, since the OS X version is up past 5.0