When working with scales, we can find the measures by using the rule of three.
From the scale, we know that 1/2 in corresponds to 1 ft, so, the rule of three is:
1/2 in --- 1ft
4 in --- x
Where "x" is the size of the line in feet repreented by the 4 in line in the drawing.
So, we cross multiply to get the equation:

Thus, this lines represents a size of 8 ft.