150 or more ppi should produce good results.
The photos do have to be cropped to a 3:2 ratio (i.e. 6x4) if they are not already in that ratio. Once you have the 3:2 ratio, there are at least three approaches:
1. Ask for borders. This will insure that no further cropping is done.
2. Resize the image to at least 1/8 inch smaller than 4x6. This means that one dimension will be more than 1/8 inch smaller. Add white area to the image to get it back to a 4x6 size. You can then ask for borderless, and, in my tests, very little of the image will be lost.
3. Find out the exact number of pixels that Costco uses for its 4x6 prints. Size the image to that exact pixel count. Have Costco make the prints, and tell them what you have done. You don't want their equipment to resize the photo. Costco does this. I don't know if anyone else does.
I ordered a few 4x6 prints of images that I had cropped a bit. I was disappointed with the results, because the prints were cropped further by the processing lab.
I would like to avoid this in the future. How can I assure that the prints will come back the way I intended? Are the dimensions important, or the file size, or do I need to stay within a particular ratio?