After using the "take four quick pictures" option in Photo Booth (the middle icon in the set of three at the lower left corner of the window), clicking on each individual photo in the "four-up" layout zooms the camera in to make that picture fill the Photo Booth window.

Note that each photo is saved individually in the Photo Booth folder.

[robg adds: If you use the iPhoto button on one of these four-up images, only the combined image is imported. As noted above, you can grab any of the originals from your user's Pictures » iPhoto folder if you want them.]