One thing to bear in mind when you do thi New Documment approach is that you will be choosing a document size and that you will need to either specify the dimensions in pixels or inches and you will need to set a ppi if you do it in inches. You'll want to set a 4x6 document to a high resolution (such as 300 ppi) so that when you resize your photo and paste it in (you also will choose/enter the dimensions for that) your document resolution should match the photo resolution (make the ppi specs the same). Setting the high res like 300 ppi ensures that your print is top quality. At that res the 4x6 document will be 1200x1800 pixels, so your image will be less pixels to "fit".
The other consideration is just the fact that although an uncropped photo from a Canon DSLR will print a 4x6 image without cropping, when you size it down to fit within the 4x6 document your actual picture dimensions can leave you with uncertain/uneven borders. Whether this matters to you is, well, up to you.