After you create your blank 4 x 6 document, go back to your picture that is now divided into 16 equal sections. Click window, arrange, tile. Now you can see both files. With layer 1 selected, use the arrow tool in the upper left hand corner to drag layer 1 to the new blank document. Click edit, free transform (Ctrl + T) and scale the image so it fits perfectly in the document. Now save that image.