`sin^4x=sin^2x*sin^2x`

`=(1-cos^2x)(1-cos^2x)`

`=(1+cosx)(1-cosx)(1+cosx)(1-cosx)`

`=(1+cosx)^2(1-cosx)^2`

Either of the last two equivalent expressions will work, as they both only involve cos to the first power.

`sin^4x=sin^2x*sin^2x`

`=(1-cos^2x)(1-cos^2x)`

`=(1+cosx)(1-cosx)(1+cosx)(1-cosx)`

`=(1+cosx)^2(1-cosx)^2`

Either of the last two equivalent expressions will work, as they both only involve cos to the first power.