Remember that "complex numbers" includes real numbers. So look for rational roots first.
Substitution show f(-1)=0, so -1 is a root. Extract that root using synthetic division.
-1 | 1 -1 0 -2 -4
| -1 2 -2 4
----------------
1 -2 2 -4 0
The remaining polynomial is . A second real root can be found using factoring by grouping.
The remaining quadratic factor can be factored over the complex numbers as
So the complete factorization over the complex numbers is