x^4-5x^2+4=x^4-x^2-(4x^2-4) = x^2(x^2-1)-4(x^2-1)
=(x^2-4)(x^2-1)
=(x-2)(x+2)(x-1)(x+1)
\(x^4-5x^2+4\)
\(=x^4-\left(4x^2+x^2\right)+4\)
\(=x^4-4x^2-x^2+4\)
\(=\left(x^4-4x^2\right)-1\left(x^2-4\right)\)
\(=x^2\left(x^2-4\right)-1\left(x^2-4\right)\)
\(=\left(x^2-2^2\right)\left(x^2-1\right)\)
\(=\left(x-2\right)\left(x+2\right)\left(x-1\right)\left(x+1\right)\)