You could use Chain Lightning against an opponent only if you were sure that the Pandora's Box gave the entire 4th level (or the entire Air magic, for example). Without the required Wisdom skill, you wouldn't know whether it was level 4 or 5, but if it gave the entire Air magic, you could figure that out based on the lower-level spells you had learned (though not always).
I interpreted it the same way. And I think everything here is quite clear.
Earlier, empty Pandora's Boxes were not mentioned in any rules. The HeroesWorld advanced rules (which account for empty Pandora's Boxes, but only those reliably identified during the game as described above).
