Op completely misses how insecure a four digit pin is for prying eyes. If I work in the same office as you, or share any space with you at all, I can pretty much guarantee I can easily sneak a glimpse at your pin when you enter it.
1. It's 6 digits now, and not 4. 2. I can also better hide myself entering the pin 3. As mentioned in other places, unlike a fingerprint or face; you can easily change pin codes. In an ideal world, facial recognition or touch id would serve as identification in addition to the pin code.
Pins are basically useless to anyone determined, no matter how much you try to hide it. There's no real bullet-proof security except for a really long password, but how many people are going to do that realistically?