This is why I love the internet. People helping people 10 years ago provided the answer I needed to install a rack onto my wife's bike. Like the original poster, the 5mm bolts seemed to large. But 4mm were too small. Standard sizes were no better. I could not find a 4.5mm, so I resorted to a more intensive search and found this thread. I cautiously followed ZPL's advice and had the same results.
Thanks to ZPL, Sheldon, and Bikeforums for providing this space, and not removing older content! Long live online forums!!