scanning photos to size

What image resolution are you scanning the photos at? What are the pixel dimensions of the scanned images. If you don't specify a resolution you may be getting just the 140 dpi Apple has set as the minimum for prints before the warning is shown (PrintWarningImageDPI). Apple does print at 300 dpi for most items.

For a 5 x 7 that would require an image of larger than 700 x 980 pixels.

However, or printing more is better and I'll scan at a minimum resolution of 300 dpi to take advantage of what Apple will print at. If you might bring the image at a size larger than then scanned side scan at a higher resoluition, up to 600 dpi

This screenshot shows the image sizes in pixels that give the low resolution warning for different print sizes.

No. Apple only provides the "standard" print sizes. You'll need to use a 3rd party online service for that.

Or crop it to 4x4, order a 4 by 6 and get out your good scissors.

That can be a bit iffy because some photofinisher software may expand the image so the smaller dimension, in this case that for the 6" width, fills the entire paper thus forcing cropping some of the image off the height. This is particularly so if there's lots of white space on one or both sides of the image even though it's cropped to 4 x 6.

If you could fill in the blank (white) space on the 4 x 6 image with color that probably would print and allow trimming down to 4 x 4.