Putting Cascade on this itself is somewhat confusing. If you cascade into a 4-mana sorcery from this, that won't have cascade, because this hasn't resolved yet.

Not that I can think of a good answer. The best is to use a pretty horrible clause from Ulamog, the Infinite Gyre and friends: "When you cast ~, until end of turn, sorcery and instant spells you cast have cascade."

That's kind of funny when you think about it, since it means this card will do nothing when it resolves.