We have the function f(x)=-x-4, and we want to find the value of f(4), this is about to evaluate a function, we just replace the x in the function for the value we want to evaluate and do the operations:
[tex]\begin{gathered} f(x)=-x-4 \\ f(x=4)=f(4)=-4-4=-8 \end{gathered}[/tex]