Let's stick with the 'low hanging fruit' as they say. Let's lock our top-right corner graphic into the top-right of our H4.
div.rounded h4{
font-size:1.3em;
color:#993300;
background-color :#fffff1;
background-image: url(tr.gif);
background-repeat: no-repeat;
background-postion:top right;
}
As a heading item, we know our H4 will always stretch to that corner, so it's the prime candidate to control that corner.
Two corners down, two to go.
The corner graphics are simple GIFs. The inside section of the graphic has been made transparent, and the outside is matched to the background color and antialiased to that color.
You might have ideas to get funky with PNGs. I'm keeping it simple for this example.