Nice.
The question is why it should costs $4, while Embargo costs $2. Its a terminal one-time Silver, like Embargo, and like Embargo it in principle effects all players the same.
The effect might be a bit stronger than Embargo, because you really can't buy from a pile that got swaped out, but on the other hand its self-inverse if you want, so you can swap the pile back. You can't undo a Embargotoken.
I'm not sure if the card makes engines more likely. It's more likely that you got all the cards you need in 12 stacks than in 10, but you can be attacked quite easily if you want to build an action/card engine and the only Village got swapped out while you are building it. You can swap it back, but that takes time. So probably BigMoney+X is a little bit too favoured by this, because it can not easily be attacked. When the games degenerate to BigMoney, Tariff itself has not much of a value.
On the other hand, an (half-)engine can expect to play a Tariff pretty fastly after it's build and continue the buildup, while BigMoney usually can wait 2-6 turns until you see a newly bought card.