If you want to print the entire photo onto a 4 x 6 stock you must select a custom size that is less than 4x6 but has the same size ratio or your original image.
For instance if your photo is 1600 x 1200 that has a ratio of 4:3 or 1.333. A 4x6 image has a ration of 1.5.
Therefore select a custom image of that's less than 4x6 that has a size ratio of 1.333. That will fit the entire photo on the stock you've chosen but will leave some white space around the photo.
Here's a photo that has a size ratio of 1.337. Printed standard as a borderless 4x6 you'll get this:
But if you select a custom size of 5.35 x 4 which is a size ratio of 1.337 you'll get this:
So it can be done but just required a bit of math.
OT