Either way, I only have a limited amount of room, but your thought did help me to figure out how to define it properly. Bullet points!
here's the new text;
Four Arms:
• You may treat two-handed and double weapons as one-handed weapons.
• You may treat one-handed weapons as having the off-hand property.
• If you would wield a one-handed weapon that normally has the off-hand trait, such as a dagger, you may wield one additional one-handed weapon that normally has the off-hand trait.
• If you are holding three or more weapons and make a weapon attack, you may choose which weapon to be attacking with. If that attack uses a main hand weapon and an off-hand weapon, you may choose which of the weapons you are wielding is which. To attack with all of your held weapons, see the Kreen Tempest feat.
The PDF has been updated. Link's the same.