The Open SQL Worksheet context menu item (right-click) for the specific connection node is not working either?
If the Tools > Preferences > Database > Worksheet > Open a Worksheet on Connect setting is not working, try Windows > Reset Windows To Factory Settings. If that does nothing for you, then it may be a matter of renaming the SQL Developer\system188.8.131.52.21 folder under a user's profile settings and relaunching the product.
Don't forget that 4.0.2 is a bit dated (not even the final 4.0.x production release) and that 4.1.2 is the most current production release.
Correct, none of the methods to open a worksheet were working correctly. Turns out we had an issue with the Temp area our users are utilizing. Once that was fixed we were back up and running.