A piece of paper large enough to accomodate 4x 4×6" prints would have to be at least 8×12" in size. A4 is 8.5×11.5" roughly, so iPhoto is organising the pics onto what would fit on an A4 sheet, I'd imagine. Maybe you need to tweak the print size settings in iPhoto, as it may be set to a default size.
Does the paper you have have any size or code? And are you sure the 'sections' are exactly 4×6"?
TBH, the printing settings in iPhoto aren't that flexible, as they don't allow you to set custom sizes for paper, or specify the individual image size. Perhaps something like Adobe Photoshop Elements might be better, unless you have access to full fat Photoshop. With Elements, you could put each image into one larger image that's the size of your paper, and scale each to fit accordingly.