If you're going to print a 4x6 then you need to make sure your image has enough detail to look sharp when you print it at that size. A fairly standard print resolution is 300 pixels per inch, so at that level of detail an image would be around 1200x1800 pixels. Visuals Clipping says that having a high enough resolution in the first place will give you loads of room for experimenting with cropping and still end up with great results. Of course the final resolution will vary depending on how good your printer is, how far away you are standing and how fussy you are about detail.