Looking at your quilt on the floor, and assuming those are 12" tiles, I'd say 4" was about right. But play with it and see what you think. You don't want to overpower your lovely quilt with a big clunky border. That being said, I have made borders larger to "stretch" a quilt to a certain size. Plus, I've done a few quilts without a border, and a dark binding, and they look fine.