The images will keep floating next to each other until they can no longer fit into what is containing them. Then, there will be a break to the next line.
So, if you want there to be four images, you could place the entire code into a div with a width that will allow 4 images to fit, then the 5th image will break to the next line.
<div id="shadowbox_images">
all your code for the shadow box images
<.div>
then in the css:
#shadowbox_images
{ width: 500px;}
or however wide you need
then add to .shadowboxlinks
float:left;