All of the following give a -4 to your attack roll:
- Attacking with a weapon you are not proficient with.
- Attacking with an improvised weapon.
- Attempting to deal lethal damage with a nonlethal weapon.
- Attempting to deal nonlethal damage with a lethal weapon.
Are all of these actually referring to the same attack penalty, or do they all stack?
I'm interested in Paizo's official stance on the matter. What follows are my guesses.
http://paizo.com/pathfinderRPG/prd/ultimateEquipment/armsAndArmor/weapons.h tml
The page above suggests that a single "non-proficiency" penalty applies for any "weapon" (improvised or otherwise) in which you are not specifically trained in how to use as a weapon. Thus, using an "improvised exotic weapon" is impossible because the "weapon" in question cannot be both exotic and improvised at the same time, even if the improvised weapon is basing its damage (and special weapon properties, such as "monk", "reach", or "nonlethal") on an exotic weapon.
http://paizo.com/pathfinderRPG/prd/coreRulebook/combat.html#nonlethal-damag e-description
The page above suggests that using a lethal or nonlethal weapon (simple, martial, exotic, or improvised) imposes an untyped -4 penalty to your attack roll, even if you are proficient with the weapon. But it isn't clear if this is simply a failure to assign greater clarity to the description or if this was done intentionally.
One could argue that, "Even if you are proficient with a scimitar, attempting to use it as a non-lethal weapon transforms the weapon into an improvised weapon for that attack because there's no such thing as a "nonlethal scimitar" in the base list of Simple, Martial, and Exotic weapons. Thus, the -4 penalty to hit with nonlethal is considered a non-proficiency penalty because you're not proficient with "nonlethal scimitars". It's like a throwing weapon being treated as a light or one-handed melee weapon to attack with in melee, but transforming into a ranged weapon the moment you throw it (thus using different stats for attack and damage and qualifying for different feat bonuses etc based on how the weapon is used... even though it's the exact same item/weapon).
Personally, I think it would make a lot of sense to just have a universal -4 non-proficiency penalty for attempting to use any weapon in a manner in you are not trained in how to use it. But, I could see potential balancing issues with traits like Blade of Mercy (http://www.archivesofnethys.com/TraitDisplay.aspx?ItemName=Blade%20of%20Me rcy) which could effectively give you proficiency to deal non-lethal damage with all slashing weapons, even if you weren't proficient at dealing lethal damage with that same weapon.
