what is the shortcut key to close the script output window in SQL developer(v4.1)

As some workaround: you can switch from current tab and come back to it with Ctrl + Tab and focus will be in the editor.

The first and main thing to learn is the magic-keyboard-sequence to take focus off the editor or grid and onto the splitter itself

Shift + ALT + F8

Then…

HOME to go full-screen output

END to go full-screen editor

UP/DOWN arrows to move the splitter

Click on the screen or hit ESC to get out of ‘move splitter mode.