Step 1 :
After factoring out $ x $ we have:
$$ x^{4}-3x^{3}+4x^{2}-12x = x ( x^{3}-3x^{2}+4x-12 ) $$Step 2 :
To factor $ x^{3}-3x^{2}+4x-12 $ we can use factoring by grouping:
Group $ \color{blue}{ x^{3} }$ with $ \color{blue}{ -3x^{2} }$ and $ \color{red}{ 4x }$ with $ \color{red}{ -12 }$ then factor each group.
$$ \begin{aligned} x^{3}-3x^{2}+4x-12 = ( \color{blue}{ x^{3}-3x^{2} } ) + ( \color{red}{ 4x-12 }) &= \\ &= \color{blue}{ x^{2}( x-3 )} + \color{red}{ 4( x-3 ) } = \\ &= (x^{2}+4)(x-3) \end{aligned} $$