Final answer:
To evaluate f(4) when f(x) = 4x + 5, substitute 4 for x in the function and solve.
Step-by-step explanation:
To evaluate f(4), we need to substitute 4 for x in the function f(x) = 4x + 5. So, f(4) = 4(4) + 5. Evaluating the expression, we get f(4) = 16 + 5 = 21. Therefore, f(4) = 21 when f(x) = 4x + 5.