I think its definitely OP.
Look not only are you paying 10 or 13 quantum, the other person is getting those in return. That is like a supernova or something. And it only costs 4??? If I an elite oty for 5, it is going to cost me 10 more to get it back? Reverse time is the closest comparison and it would only cost the price of the creature again, forgetting that the opponent actually gets the quantum in return.
I think you need to lower the release cost. Only a rainbow deck will be able to afford this and i just think it is unfair to give 10 or 13 quantum to the opponent. I say lower it to 5 and 8 or something.
Most mono decks with 3 or more pillars on the field will be able to pay the ransom within the 3 turns easily. But the question is.. are you willing to pay for it?
Scenario 1: Charger gets kidnapped, you wait 3 turns and let it die
Scenario 2: 20/15 golem gets kidnapped
now what chief? The choice is in your hands
And note: the released creature is EXACTLY the same as when before it got kidnapped, so any buffs WILL be kept after release. Unlike rewind, which completely resets the creature. (This also costs a lot more to cast)
AND the quantum paid is NOT converted to darkness (although i'm thinking of making it so), so the Kidnap user may not even have a use for those quanta