Having installed the HP software, I am able to see, in Page Setup, the whole range of papers supported by the printer when I pick it from the list of printers. I pick 4x6 or 4x6 borderless. Then I go to Print ... and choose Standard Prints. If I pick 4x6, then instead of a preview in the little pane on the left, I get text that reads, "The selected print size will not fit the current paper size". I can pick 3x5 or 2x3 and the preview looks right, a little image in a bigger (ie 4x6) shape. Why does iPhoto think a 4x6 print is too big for 4x6 paper?
Note that this occurs even after I have cropped the original photo from 3:4 to 4:6. Frankly I am amazed (a) that iPhoto does not offer to do that automatically at print time, since most cameras shoot 3:4 but many people will want to print 4:6, and (b) the iPhoto help does not clearly explain that you will need to do this to each and every 3:4 image that you want to print 4x6. The software that came with the printer (Mac and Windows versions) will crop at print time without editing the original image, and in the majority of cases that is exactly the crop you would do (slowly and manually) in iPhoto.
I have tried with all three orientations in Page Setup, with and without the "one photo per page setting in the Print... dialog (which seems to change the orientation of the image as well), also with image size as low as 90% in case for some reason it was just over. None of this helped. And by the way you cannot print when you get that error message, hoping that you are just a few pixels over the edge: iPhoto will not send to the printer.
As suggested in this thread, I found that if in Print ... I choose Full Page instead of Standard Prints, and I set margins to zero using the slider that appears, I do get a preview, but it shows some white margin along one of the 4 sides. However, to my surprise, when I print this, the print comes out borderless.
All I can assume is that the cropping which iPhoto is doing (constrained to 4x6) is not actually cropping to that size, or else what iPhoto thinks of as 4x6 does not quite correspond to what my HP Photosmart 2610 thinks of as 4x6. The second seems more likely. Ie if I crop to iPhotos 4x6, and choose 4x6, it would seem that it must fit, except that the paper size is coming from the printer, and maybe HPs 4x6 is just a tiny bit shorter than what iPhoto expects, so iPhoto says it will not fit, and iPhoto then also shows a white edge when I work around using the Full Page setting.
This is very embarrassing for someone who is used to evangelizing Macs and only barely putting up with Windows ...