[tex]\bf f(x)=\cfrac{2x^4}{5}\implies f(x)=\cfrac{2}{5}x^4\implies \cfrac{df}{dx}=\cfrac{2}{5}\cdot 4x^3\implies \cfrac{df}{dx}=\cfrac{8x^3}{5}[/tex]
notice, the 2/5 is just a "scalar" value, so you can just get the derivative of the variable part and slap the scalar as its factor.
