You suffer a -4 penalty to disarm a target if you are unarmed. Does that mean if you have improved unarmed strike you no longer suffer that penalty?
From the IUAS feat:
You are considered to be armed even when unarmed—you do not provoke attacks of opportunity when you attack foes while unarmed. Your unarmed strikes can deal lethal or nonlethal damage, at your choice.
So you're no longer considered unarmed, and don't take the penalty.