4" ducting is a commonly used size for ventilation systems in various applications, including HVAC, kitchens, bathrooms, and other ventilation needs. When purchasing 4" ducting, it is typically sold by the length or per foot. Each foot of ducting refers to a one-foot section of the duct, and you can purchase the desired length based on your specific requirements.

It's important to note that ducting is often sold in rolls or bulk quantities, so you may need to specify the total length you require when making a purchase. Additionally, consider the material of the ducting, such as flexible or rigid, depending on your installation needs.

When purchasing 4" ducting per foot, you can select the desired quantity of feet based on the length of ducting you need for your project. For example, if you require 10 feet of 4" ducting, you would purchase a quantity of 10 when ordering.

Remember to consult local building codes and regulations, as well as any specific requirements for your ventilation system, to ensure that the ducting meets the necessary standards for your installation.