If α,β,γ and δ are roots of a biquadratic equation  ax⁴ + bx³ + cx² + dx + e = 0, then

α + β + γ + δ = -b/a    ..................1

αβ + βγ + γδ + δα = c/a ................2

αβγ + αδγ + αβδ + βγδ = -d/a .......3

αβγδ = e/a  .................................4

Now solving these equations, we can find the roots of the biquadratic equations.