Factor.

(y-1)⁴ - (y-1)²

you can factor (y-1)² from both, resulting to:

(y-1)² [(y-1)² -1]
since both terms inside the bracket are perfect squares, we can still factor them, resulting to:
(y-1)² [(y-1)-1][(y-1)+1] or
(y-1)² [y-2][y]
rewriting,
y(y-2)(y-1)(y-1)

hope this helps~ :)