Usually the keychain password is the same as the Administrator one for the system. If it is different, yet you can't remember it, the password won't be accessible (for security reasons). You'll need to delete your keychain file and either start-over, or if you have an older version of the keychain.login file, replace the existing one and try again (you might get lucky if that version has a PW the same as your Admin PW).
If the PW for Keychain is the same as your Admin, open Keychain Access in your Utilities folder and via the KA menu run "Keychain First-Aid">repair.
Hard to understand how a routine update would corrupt the keychain. Not saying it didn't happen, just "hard to understand how...."
Have a look at this
timely MacWorld article. If resetting the keychain does not work you can always replace the newly created keychain file with the older copy.
By "older copy" I mean the one installed prior to running the reset. As the article mentions, when you reset the keychain, the existing keychain file is replaced with a fresh copy. By chance, if you remember the original password, you could always go back to the initial copy by renaming it to keychain.login and have access once more to your various web site user names/passwords.
Yes. When you downloaded the Safari BETA 4 version there was an UNinstaller package. To to your Downloads folder and look for this file; Safari4.0BetaLeo.dmg Double click the .dmg, then double click the uninstaller pkg.