CAT 2018 Number System — Previous Year Question

$X$ and $Y$ are the digits at the unit's place of the numbers $(408X)$ and $(789Y)$ where $X \ne Y$. However, the digits at the unit's place of the numbers $(408X)^{63}$ and $(789Y)^{85}$ are the same. What will be the possible value(s) of $(X + Y)$?