Question 834668
Use a substitution,
{{{u=x^2}}}
{{{u^2=x^4}}}
{{{v=y^2}}}
{{{v^2=y^4}}}
{{{(x^4)-5(x^2)(y^2)+(4y^4)=u^2-5uv+4v^2}}}
{{{(x^4)-5(x^2)(y^2)+(4y^4)=(u-4v)(u-v)}}}
{{{(x^4)-5(x^2)(y^2)+(4y^4)=(x^2-4y^2)(x^2-y^2)}}}
{{{(x^4)-5(x^2)(y^2)+(4y^4)=(x-2y)(x+2y)(x-y)(x+y)}}}