On your way to pick up your photos at the print shop swing by your local thrift store and get a board book to use as a base. The book should be at least 4x6 inches in order to fit the prints. I cut mine down to exactly 4x6 inches to match the prints using a saber saw with a fine blade. You could also use a utility knife to cut one page at a time. I spray painted the binding black to hide the old book title.
As another alternative (which I'll probably try on my next book) you could glue the photos onto individual pieces of cardboard and then bind it yourself or take it to a print shop and get it spiral bound. Using an old board book as my base spared me from dealing with binding the thick pages but it ended up making it difficult to close the book due to the extra thickness of the four prints between pages. This isn't all bad though because it makes it much easier for a toddler to turn the pages since they stand apart.