Sean, I'm afraid the aspect ratio of your recorded image is what it is. I believe you have a couple of options though, one, you can use software to edit your image, cropping it to meet the aspect ration of the 4 x 6 or you can find a print house that offers prints matching your aspect ratio. For example
Winkflash offers prints of 5 x 6.6, 6 x 8 or 8 x 10.6 which all are more closely matched to your aspect ratio.
<disclaimer> I am not affiliated with Winkflash, there are many other online print houses offering the same sort of thing. </disclaimer>