Hi,
I think this selector is not right:
h1, h2, h3, h4 p { }
because h4 p {} means any p inside an h4 which is an unlikely nesting.
In any case the p is making it not affect the h4, if you remove the p it should fix it.
h1, h2, h3, h4 { padding: 0;margin: 10px 15px;}
Hope it helps,
Joe