The expression given is:

$f(x) = 5x - 9; \, x = 4$

To find $f(4)$, we substitute $x = 4$ into the equation.

$f(4) = 5(4) - 9$ $f(4) = 20 - 9$ $f(4) = 11$

Thus, $f(4) = 11$.

Would you like any additional details, or do you have any questions about this solution?

Here are 5 related questions for further exploration:

1. What is the slope of the function $f(x) = 5x - 9$?
2. How would the value of $f(x)$ change if $x = 6$?
3. What does the graph of $f(x) = 5x - 9$ look like?
4. Can you find the x-intercept of the function $f(x) = 5x - 9$?
5. What would be the inverse of the function $f(x) = 5x - 9$?

Tip: For any linear function in the form $f(x) = ax + b$, the slope is $a$, and the y-intercept is $b$.