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?