The toga button is an elongated oval, taller than it is wide. There is no separation in the lines of shading at the right of the button.
The toga button is a flattened oval, wider than it is tall. Most Type IVs show a slight separation in the lines of added shading at the right of the button.
This is the second key distinction separating the Type III and Type IV stamps. The extra shading should be present to be a Type IV stamp. This is a difficult distinction from the examples shown here. To this observer, there is clearly a line defining the top of the toga button on the type III stamp and no such line on the type IV.
All the Type III examples are from the same scanned stamp from plate number 146, and all the Type IV examples are from the same scanned stamp from plate number 762.