如何在Safari 3和Safari 4中进行测试？

I only have one computer (running OS X), and need to test in both Safari 3 and Safari 4. While Safari 4 is the "official" release, I don't believe it has been pushed in Software Update yet, and therefore probably hasn't been adopted by most users. Even if it has been pushed, it's still a safe bet that many users haven't updated.I only have one computer (running OS X), and ne