As I understand it, Apple has to use the new webkit and built it into a newer Safari 4 Beta/Final. You as the comsumer can't change it.
Correct me if I am wrong.
This is one of those areas where, if you have to ask, you probably shouldn't do it.
You can run against a newer build of WebKit if you really want to. It's possible, and the WebKit site has details for downloading the nightly builds (that is, the very latest and greatest version) and running Safari with those nightly builds. Those details assume that you're very geeky, comfortable with Xcode and Terminal, and don't mind the idea of something breaking in some very messy way.
However, running against a nightly build of WebKit is probably not something that the average person really does want to do. It's brand-new untested code, so it could break something for you. This is the bleeding edge of software development. Living on the bleeding edge virtually guarantees that you are going to bleed at some point.
Regards,
Nadyne.