D4 means double 4 inches (4 inches multiplied by two), which is 8 inches wide plus the nailing ham and interlocking details. D5 means double 5 inches, which would make it 10 inches wide plus the nailing ham and interlocking details. Whether you select D4 or D5 for your home will depend on your personal tastes and preferences. Fortunately, our team at Legacy Service will help you choose between the two as well as a myriad of vinyl siding types and colors so that you can build the home of your dreams.