Full version of jQuery 1.4 (minified) was released some time ago. Also, the final version ofjQuery 1.4 API is now published.
Go ahead and give it a try. And remember to keep yourself updated by the action and buzz around jQuery 1.4 by following The 14 Days of jQuery event.
http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.js
http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js