How do we plot $u(4−t)$, where $u(t)$ is a step function? How do we plot $u(4−t)$?
$u(t)$ is a step function:
$$u(t)=\begin{cases} 1&\text{ for }t \ge 0,\\
0 & \text{ for }t \lt 0.\end{cases}$$

$u(4-t) = 1$ for $4-t\ge0$ so for $t\le4$
$u(4-t) = 0$ for $4-t\lt0$ so for $t\gt4$
Here is the plot:

