Answer
$(-\infty, 4)$
Refer to the graph below.
Work Step by Step
By adding $-1$ to both sides we get
$$
x+1\lt 5 \Longrightarrow x+1-1\lt 5-1\Longrightarrow x\lt4
.$$
In terms of intervals, $x\in (-\infty, 4)$.
To graph, plot a hole at $x=4$ then sahde the region to its left.
