The line in the hair at center is slightly shorter than the line below it.
The strong line at center is longer and has been strengthened as well as extended.
This is the main distinction separating the Type III and Type IV stamps. This line must be stronger and longer than the line below it to be a Type IV. The strengthened line may not always be as obvious as in this example and, if so, check the other areas for confirmation of type.
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.