Just checked. The default passcode on my iPhone 8 is a 6 digit numeric code, not 4. although you can switch to 4 it seems.
Even 4 digits could be enough, given that iOS enforces (very long) delays after a few failed attempts.
Even 4 digits could be enough, given that iOS enforces (very long) delays after a few failed attempts.
This is, apparently, the thing that GrayKey is able to bypass.