(4) Where the substance exceeds 20 kilograms by weight, or 400 liters by liquid volume, the person shall receive an additional term of 15 years.
In computing the quantities involved in this subdivision, plant or vegetable material seized shall not be included.
The conspiracy enhancements provided for in this subdivision shall not be imposed unless the trier of fact finds that the defendant conspirator was substantially involved in the planning, direction, execution, or financing of the underlying offense.