When a triangle is translated, the dimensions of the triangle remain unchanged, so the side lengths do not alter through translation.
Given that triangle XYZ is translated down 4 units and to the left 8 units, the side lengths will remain the same before and after the translation. Therefore, the length of side X'Y', after the translation, will still be equal to the length of side XY, which is 10 units.
Thus, the length of side X'Y' is 10 units.