But how do you know if that spell is in there or not? For example, I take an empty Pandora's box (but there was some magic inside), or I only get the first two levels, and who knows if the 4th one was there. In short, maybe I'm missing something, but this point is unclear and could lead to ambiguities.
I suggest renaming it to "if any of your heroes learned this same spell in a Pandora's box, it can be used by level 4 magic specialists against players". The same applies to DD, Fly, and TP.