8 is sane here. The + operator adds numbers. It converts the string to a number implicitly in the process. it takes "0x4zz5" and tries to parse as much as possible. Since "z" is not a valid hexadecimal digit, it stops at z, and reads it as 0x4.
Well, I'd say it's never better to "try to parse as much as you can" and when encountering such an error simply silently give out a nonsense answer.
It's the computer equivalent of a bozo worker who does something random everytime they don't know how to do their job, and then tries to hide any consequences while whistling innocently.
It's not equivalent at all. It is an expected, clearly-defined behaviour - automatic, silent type conversion, with clear rules on how that type conversion works.
I'd bet 1-to-10 that if someone uses '+' operator then that line of code is never expected to encounter "0x4zz5" there. And I'm pretty sure that this addition is not meant to return a number exactly 4 less if it encounters "0z4zz5" instead of "0x4zz5". There is some theoretical chance that it does, but really, it's a lottery-type chance.
The whole total function has something that it's expected to do, some value that it returns in the end - and if it can't do that then any returned value is useless and wrong anyway; and automatic, silent type conversion is the worst possible choice of action, since it not only doesn't work, returning a value that you didn't intend (as other options); but in addition makes the actual bug hard to find and potentially corrupts a lot of stored data before you find the bug.
Of course, it is better to manually convert.