Jeff (The Brown-Eyed Albino)
It turns out that when you install Safari 4, it overwrites any previous versions as well as the Webkit frameworks. So do you want to keep the stability of Safari 3, but still test out the latest beta? In comes www.BeatnikPad.com. They've put out a quick and easy how-to guide that will give you the fix you're craving. You're going to need some basic Terminal skills, and an older copy of Webkit to do this, which shouldn't be very hard to find.
Check it out at BeatnikPad.com