Paul_o_b - 10:55 pm on Jan 1, 2013 (gmt 0)
You have made the h4 an inline element so that means it is only as long as the content that it holds. A border on an inline element will only match the content.
If you want the border to go full with of the container then set the element to display:block instead.
Note that you can't set vertical margins on inline elements and that vertical padding will not increase the height of the line but spill out over other elements.