if your talking about 4x6 BORDERLESS printing, then select the photo you want a 4x6 of, go to the Page Setup (in the File menu) and look for 4x6 and/or the 4x6 borderless. I have an Epson 1270 and it does not offer 4x6 borderless, but when I use my Canon i960 I'm able to 4x6 borderless and it also shows up in the menu.
Then print your photo using iPhoto's option located in the lower bottom of icons.