Nearly every browser has "hidden" preferences. Options that you can set by running a command, editing a file, or changing an entry here or there. The Safari 4 Beta, which has only been out for a few days, is no different.
A way to restore the old tab bar.
Turning off the auto-complete search bar.
...and much more.
I am not sure why you'd want to change some of these preferences (as I feel they are some of the best features of the new version of Safari), but they are there, you can if you want, and Caius shows you how.
Source: Safari 4 Hidden Preferences.