It's a matter of the differences in the resolution that the image was scanned in and the resolution it's being printed in. It takes a while to get your head wrapped around what's going on but try thinking of it this way. You start out by scanning an image that's 4"x4". You tell the scanner to scan the image at 400 DPI (Actually it should be designated as LPI, lines per inch). The resulting file will be of an image that's 1600x1600 pixels. Scanners don't know from inches, they only know pixel dimensions.
Now when you go to print the image your printing or editing software may be set to print at 200 DPI/LPI. So every 200x200 pixel area of your image will occupy a 1"x1" area of your print. As a result your 1600x1600 pixel image makes a print that's 8"x8" instead of your original 4"x4" size.
Inkjets like around 300 DPI/LPI to make good quality prints. If you want your print to be the same size as your original, or smaller, set your scanning resolution to 300 DPI/LPI. Then in your printer or editing software check the document size and set the resolution to 300 DPI/LPI without resampling the image. This should give you a print the same size as your original.
If you haven't already, spend some time reading up on your scanner and printer or editing software user guides.
