We've been having this discussion in a game for a while. Disarm says if you attempt it while using an unarmed weapon, you suffer a -4 on the check. Simple enough. I'm under the impression this also counts even if you are a monk doing an Unarmed Disarm, but does the penalty also apply when using a Cestus? After all, it says your Unarmed Strikes can do Piercing and you're treated as armed when avoiding attack of opportunities.
So, do you suffer a -4 when trying disarm while wielding a Cestus?
Cestus: The cestus is a glove of leather or thick cloth that covers the wielder from mid-finger to mid-forearm. The close combat weapon is reinforced with metal plates over the fingers and often lined with wicked spikes along the backs of the hands and wrists. While wearing a cestus, you are considered armed and your unarmed attacks deal normal damage.
Attempting to disarm a foe while unarmed imposes a –4 penalty on the attack.
By the first quote, you are considered armed, period; therefore you don't trigger the second quote.