To understand the steep of a graph. It is better to visualize it in slope-intercept form, that is:
y = mx + b
here, m is the slope that defines the steepness of the graph. The higher the value of m, the steeper will be the graph.
b is the y-intercept that shows the value of y when x is zero. If b is also zero, that means the y-intercept lies at the origin.
Now, if we see both the graphs and map it to the slope-intercept form. We get
m1 (slope of y =2x) = 2
m2 (slope of y =4x) = 4
As the slope of y=2x is less than the slope of y = 4x. Hence, y=2x is flatter than y=4x.