You have the following function:
y = 4x + 5
In order to determine the slope of the line, take into account the general equation of a line:
y = mx + b
where m is the slope and b the y-intercept. By comparing the previous equation with the given equation, you can notice that m = 4.
The slope of any parallel line to the line y = 4x + 5 has to have the same slope, hence, the slope of a parallel line is also m = 4