Original coordinate:
Z (0, 4)
Translation being used: (x, y) > (x+1, y-8)
Since your going one unit to the right, then you would be adding since your x is increasing. Since your going eight units down, then you would be subtracting from your y since it's decreasing.
X coordinate translation:
(1, __)
Y coordinate translation:
(__, -4)
New coordinate:
(1, -4)
HOPE THIS HELPS! ^_^