if the image isn't big enough to crop to 4x6 and is not a 3:2 aspect ratio, you may be out of luck. Stretching a square image to fit a rectangle space is always going to distort it. If it *is* big enough to crop, figure out what you can cut out fo the picture without missing it, and do it.
Another question, while I'm thinking about it: are you going to print this yourself, or take it somewhere? (walmart, walgreens, a photography shop, etc.)? One thing you can do without distorting the picture is to change it's density rather than it's physical size. get into the resize dialogue box, uncheck the "resample" box, and play with the DPI settings. change it, click ok, then try to crop your 4x6 image out of it. use the crop tool's pre-set 4x6 vertical or horizontal selector to make sure you get the right size. I'm not sure what the exact limits would be but I try to stay around or above 250DPI. 300DPI is standard for printing, I think.
Nate