I’m new and unexperienced, but for what it’s worth I am going to try to give this a wack.
I would try to make the four smaller div’s equal to 1/4 th the size of the wrapper. So, if the width with padding and borders are 50 px each I would make the wrapper 200 px total. Also if you are wanting to center the main div in the page. I would use an auto margin for left and right.
Another thing I would take a look at is the technique for making each column equal length. To do this take a look at the technique of wrapping each div in a large wrapper and using absolute positioning. I found this technique in another post on this forum. Here is the url for the previous post http://css-tricks.com/forums/viewtopic.php?f=2&t=2370
Again I’m not much experience but I’m trying to learn. Hope this helps you as much as it helped me when I found it.