How do I put 4 elements, one in each corner of the div? I can't use regular absolute, because the div isn't the same always. And relative, can be tricky.
For the upper two you just float them left and right and for the lower two put them inside another div which has the same width as the main div. Position this div to the bottom of the main div. Then inside it float the lower two elements to left and right.
If you're trying to create rounded corners, then I suggest you use a tutorial from A List Apart, like this one: http://www.alistapart.com/articles/customcorners/