Apple doesn’t seem to offer downloads of older versions of Safari, and I’m not even sure they would install on Mac OS X, as Safari seems to be part of the OS like Internet Explo
Multi-Safari works for 10.9 (Mavericks), but only the Snow Leopard Safari 4 version.
Michel Fortin’s Multi-Safari looks like a good bet. 3.0.4 worked great for me on Snow Leopard.
(Bonus question: did Michel Fortin work on Markdown?)
Browserstack seems to be the fastest way to test versions 4/5 of Safari at this point without spinning up a VM.