Hi Sean,
there are a variety of programs u can use. The impt point to note is that the program must be able to show your pics in cm / inches and must be able to resize.
Microsoft Paint in win2k: If u can open ur pic in Paint, lick on Image -> Attributes. U will see ur pic's attributes, and u can resize to the 4"x6".
Adobe Photoshop: This is a commercial software, very powerful and can do your task easily. But u need time to learn and it's quite xpensive. There's an automate function which can resize all ur pics at once if u put them in a folder.
Corel Draw: Another commercial graphics program. It can achieve your goal, but I'm not familiar with it.
