Spring Type 4 Blue. Additional spring for pantograph top 4C. Allows safe and balanced mounting of a heavy light fixture to the Pantograph of a Bogen Rail System. For loads over 26 lbs. When choosing the additional spring(s)for your pantograph, you will first need to know exactly which model of pantograph you have, in order that you can select the spring of the correct length. If you have the new compact pantograph, the TOP 2 version has a red notched ring, while the TOP 4 version has a blue notched ring.