@takuya_matsumoto, Thanks for posting in Q&A. Based on my researching, Apple revealed that all newer devices running the newer version of the operating system will be required to upgrade from a four-digit to a six-digit passcode. Passcodes remain optional, and users can create a more complex alphanumeric password, but six digits will be the minimum.
Note: Non-Microsoft link, just for the reference.
So we now can only set at least 6 digit passcode.
Hope the above information can help.
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