Good question! I'm going to say, yes, you have to drop the fourth item in this situation. Seems the most consistent. However, if you disagree, feel free to apply the penalty to actions not yet taken this turn.
Thanks for the reply, Philip!
I agree that being forced to discard the +1 item is the most consistent step to take in this situation, especially considering the deputy penalties for Hess (discard a card) and Goering (discard an item tile) can also result in a conspirator having to forfeit an asset.