To find $f(0)$, substitute $0$ for $x$ in the function $f(x) = 4x$.

$f(0) = 4(0) = 0$

So, $f(0) = 0$.

Would you like more details or have any questions?

Here are 5 related questions for practice:

1. What is $f(2)$ if $f(x) = 4x$?
2. How do you determine $f(-3)$ using the same function?
3. What would be the value of $f(x)$ if $x = \frac{1}{2}$?
4. If the function changes to $f(x) = 5x + 2$, what is $f(0)$?
5. Can you explain what the slope of the function $f(x) = 4x$ represents?

Tip: When finding $f(a)$ for any value $a$, substitute $a$ in place of $x$ in the function rule and simplify!