\(a,4x^4+1=\left(4x^4+4x^2+1\right)-4x^2\)
\(=\left(2x^2+1\right)^2-\left(2x\right)^2=\left(2x^2+2x+1\right)\left(2x^2-2x+1\right)\)\(b,x^4y^4+4=\left(x^4y^4+4x^2y^2+4\right)-4x^2y^2\)
\(=\left(x^2y^2+2\right)^2-\left(2xy\right)^2=\left(x^2y^2-2xy+2\right)\left(x^2y^2+2xy+2\right)\)