With that amount of information you won't get much help.
First of all, what program do you use to print the picture?
Second, what printer are you using?
Many printer have a setting that can center the picture horizontal as well as vertical. Your printer dialog will tell you.
Also, if you print in 4x6, your printer might have a setting where you can fill out the paper (resulting in a slight overscan) .
I for myself print my pictures on 4x6 with a Canon printer and Irfanview as the program of choice. They result in a slight overscan, meaning that the printed picture is slightly bigger than the paper. In my case this is called "Borderless printing".