Actually, a place like that would likely print sRGB so changing to that color space would be the thing to do. I'd bet that 99% of the people walking in the door don't know about color spaces - it's a retail store. I use a Fuji Frontier at a grocery store and had great luck, it's sRGB all the way. Pro labs can work in Adobe RGB, but places like this aren't set up for it.

I'm using CS and can't remember if it's different with 6.0 - but I use the crop tool when downsizing for a smaller print like a 4x6. Since the file size will be pretty small anyway I'd use 300 for the resolution. Set the crop tool at "6in" wide and "4in" high with a resolution of 300dpi. Drag the crop tool over the image and the file is ready to go. Don't forget to "save as" to a different place.