Why does the CSS property (z-index) have a maximum of 2147483638, instead of "4", as per the W3Schools?

Answer 1

The max value of z-index has always been 2147483647 (max number on a 32 bit system). I believe your misunderstanding is that W3School's examples shows only up to a z-index of 4. But in practice, the min and max value of z index is -2147483648 - 2147483647.