I am lucky to have had both of my grandmothers meet my first born child. In both of those instances, we took a four generations photograph. Those are photos that I treasure, and I'm sure my daughter will come to treasure them as well. Of course, when you have a photo like this, you are going to want to scrapbook it. And, better yet, scrapbook it four times! That way, each member of the family that was pictured can have a copy of the photograph that is displayed in a beautiful way.
For my grandmother, I would probably create a layout that she could frame and hang. Obviously it would be difficult to create a scrapbook with just one photo, so the beauty of heritage scrapbooking is that you can scrapbook one layout, and then frame it as a gift. For my Mom, I could do the same. However, for my daughter, and myself, I will just add these layouts to current albums that I am working on. It will be nice for her to have a copy in her own baby album, and for myself to have a copy in my family album.
When scrapbooking a photo like this, you can get creative with the number four. The title of your layout could simple be "four generations." Since you are going to have just one photo as the central focus of the layout, it might be nice to matte the photo four times. For example, choose 4 different colors of cardstock that coordinate. Cut each one a little bigger than the last so that you can matte the photo with four mattes. You could even write the name of each person in the photo on one of the mattes.
You can repeat the theme of four on various parts of the page. For example, use four flowers as embellishments. Tie four ribbons, in four different colors around the main page of the layout. Cut the ends short so that you don't have too much extra ribbon. You can use four journaling tags to tell more about each person in the photograph. There are countless ways that you can repeat the theme of "four" on your scrapbook layout.
Having a photo like this is special, so you want to think of a unique way to scrapbook it that will give respect to the photo, and make it interesting to look at.