The natural combination would be the 4-key <kbd>ctrl</kbd>+<kbd>alt</kbd>+<kbd>shift</kbd>+<kbd>enter</kbd>, though I don't know if there's a subtlety involved in doing a 4-key combo. I'd alternately suggest <kbd>ctrl</kbd>+<kbd>space</kbd>+<kbd>enter</kbd>.
The natural combination would be the 4-key ctrl+alt+shift+enter, though I don't know if there's a subtlety involved in doing a 4-key combo.
I'd alternately suggest ctrl+space+enter.