HELP!!! Unable to launch safari.

Greetings,

Did you restart your Mac after installing Safari 4? If not, it definitely won't work, so you would need to do that. If you did restart, where did you attempt to launch Safari from: the Dock, or the /Applications folder? What happened when you did this, exactly?

Also, don't try to delete and reinstall applications willy-nilly; it won't solve your problem and unless you know what you're doing, you'll create a bigger problem.

What version of Safari is currently installed on your Mac, if any? What version did you attempt to install, exactly?

You need to remove Safari 4 before you can install a previous version, but you can't just delete the application and expect to be able to install something else. You have to do this:

Download Pacifist.
Go to /Applications and delete Safari.
Go to /Library/Receipts and delete the Safari403Upd.pkg file.
Open the Safari installer you previously downloaded (3.1.2, or 3.2.3) with Pacifist.
Select the top-level package icon in the Pacifist window for that installer and click Install in Pacifist.
When asked by Pacifist, choose to overwrite or replace all files that already exist.
Quit Pacifist when done, and restart your Mac.

You couldn't have lost your bookmarks, because Safari doesn't touch them during an install. They should still be in Home/Library/Safari, in the file Bookmarks.plist