Welcome to the world of digital cameras and the non- compatible world of
digital printers. Although they are supposed to work well together, they
don't and your results are typical and expected.
The reason is that digital cameras produce an image with an aspect ratio of
4 (length) x 3 (width). Divide 4 by 3 and you get 1.33. This is the aspect
ratio.
You want this image printed 6 (length) x 4 (width). Divide 6 by 4 and you
get 1.5.
You can't fit 1.33 into 1.5. Impossible.
So what are the options.
You stretch 1.33 to 1.5. As you wrote, the proportions are now out of lines
and you get an ugly picture.
The other option is you let the software print the entire image with no
stretching, but now it will print a smaller and correct image within the
paper size you selected, but to fit it in correctly you will have to accept
white edges around the image.
There is a third option.
For this, you will need to purchase photo editing software or download a
free one.
A very good one if you are into printing is Qimage, you can download the
demo here and try it:
http://www.ddisoftware.com/qimage/
and you can also download the manual and learn how to use it.
The other would be Microsoft Digital Image 9. Lots of fancy printing options
with it but a poor manual.
They both provide this third option to crop your image to a specific size,
your specific size being 6 x 4. So you will have to remove a little bit from
the top and a little bit from the bottom of your image and then it will fit
6 x 4 perfectly. Qimage does this automatically. Image 9 gives you a
guidelines on the screen to do it.
Here is another site, explaining why you can't fit your 4:3 images on 6 x 4
paper. You can also download the free software from this site to
automatically crop your image to fit. You save the cropped images (they are
in effect automatically saved for you) and you then print them with the XP
Wizard. They will fit perfectly.
http://ekot.dk/programmer/JPEGCrops/