Rewrite y4 as (y2)2.
Let u=y2. Substitute u for all occurrences of y2.
Factor u2+3u-4 using the AC method.
Consider the form x2+bx+c. Find a pair of integers whose product is c and whose sum is b. In this case, whose product is -4 and whose sum is 3.
Write the factored form using these integers.
Replace all occurrences of u with y2.
Rewrite 1 as 12.
Since both terms are perfect squares, factor using the difference of squares formula, a2-b2=(a+b)(a-b) where a=y and b=1.