The function defined by \(f(x) = \left\{ \begin{array}{cl} x + k & \text{if $x < 4$}, \\ 2x - 3 & \text{if $x \ge 4$} \end{array} \right. \)
has an inverse, and the inverse is defined for all real numbers. Enter all possible values of \(k\) separated by commas.
