What's the best way to place 4 pictures on a A4 sheet using the mac?
You can do this using Preview. Here's how I do it (on Lion, but I'm pretty sure it was much the same on Snow Leopard).


Select the 4 images in Finder
Open with Preview
Select the 4 images in the Preview sidebar
From the menubar, select File > Print...
Change the dropdown half-way down the page to "Layout"





Change "Pages per sheet" to 4 (you should now see the 4 images together on the single sheet on the left)





Select "Save as PDF..." from the PDF dropdown in the bottom left




You can also customise the paper type, layout direction and approximate scale of each picture to whatever suits best.