OK. Yet another Iron Discipline question. I'll just restate the card for helpfulness.

ACTION: Target one unit. Until the end of the turn, cancel any other action that targets this unit unless the action's controller pays an additional 4 resources (per action).

OK. So my Chaos opponent plays Seduced by Darkness on my Peasent Militia

Action: Corrupt one target unit.

As a response, I play Iron Discipline on my Peasent Militia

OK. Now.

If my opponent doesn't have the 4 resources to play the increased cost... were does the card go?

Does it just remain in his hand? As he can no longer pay the cost to play the card?

Or is it now classed as an Illegal Target under the FAQ rules and the card fizzles into the discard pile?

Seduced was paid for, target chosen, etc., it will hit the discard pile unless opponent pays the 4.