Step 1 :
After factoring out $ x $ we have:
$$ x^{4}+x^{3}+100x^{2}+100x = x ( x^{3}+x^{2}+100x+100 ) $$Step 2 :
To factor $ x^{3}+x^{2}+100x+100 $ we can use factoring by grouping:
Group $ \color{blue}{ x^{3} }$ with $ \color{blue}{ x^{2} }$ and $ \color{red}{ 100x }$ with $ \color{red}{ 100 }$ then factor each group.
$$ \begin{aligned} x^{3}+x^{2}+100x+100 = ( \color{blue}{ x^{3}+x^{2} } ) + ( \color{red}{ 100x+100 }) &= \\ &= \color{blue}{ x^{2}( x+1 )} + \color{red}{ 100( x+1 ) } = \\ &= (x^{2}+100)(x+1) \end{aligned} $$