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:
$$u(t)=\begin{cases} 1&\text{ for }t \ge 0,\\
0 & \text{ for }t \lt 0.\end{cases}$$

The unit step function has a single transition when the argument is zero.
Check where (4 - t) is zero, that's where the function switches from 0 to 1 or from 1 to 0. To know which transition it is, try a value for t less than or greater than 4, for instance t = 0.
