The 4 ½-inch measurement is the length of the tool, while the 1mm is the distance across the very end of the tip. The idea behind this tool is to use the 1mm tip to measure the length of, and then make the 90-degree bends on, the wire prong being created to hold a cut stone within a wire design.
Due to the fact that the necessary lengths of prongs are determined by the size of the stone being set, there are several different sized 'prong making' pliers available. Larger stones need prongs longer than 1mm, so if one were to purchase all of the sized pliers available they would have a choice.
Answer contributed by Dale "Cougar" Armstrong