Some might refuse to write 4s (vs U+1FBF4) and 8s like that as they require two strokes or retracing.

... what?

Can you explain how this is an issue at all and a point of concern for something like USPS?

The (arbitrary?) 9 segment number examples from the Russian Postal Codes that were linked and were questioned as to why the USPS did not adopt them are ambiguous depending on how one chooses to fill out the segments of the numbers. For example 4 may be seen as 9. 8 is slightly less an issue, but amounts to 1 segment away from 0 and 9. If a machine or any and all perfectly rule following humans would be writing and interpreting the zip using 10 different characters mapped to the Arabic numbers, then it would be more optimal to try to maximize the mutual distance between them such as an anti-reflected binary code, which would be an increasingly challenging exercise with more segments that could start at 7.

> For example 4 may be seen as 9

Except no, specifically because 9 has a slanted ending compared to 4. If some idiot writes the symbols whatever they wants instead of the reference then it's the the problem of the idiot.

And any way, most people would write it properly which means more mail would be processed efficiently and require less human intervention.

You think handwriting is easier to recognize than digits of a standard form and size?

No