Rick, re the calculations, I would look at it differently. You're really
cropping pixels, not inches, so set aside the inch measurements for the
moment. If your image is 2500x1500, that gives you 3,750,000 pixels to work
with (2500x1500). If you reduce that number of pixels by 25%, you'll have
2,812,500 pixels remaining. The trick is to find the pixel dimensions
(length in pixels, width in pixels) with the same aspect ratio as the
original that will give you 2,812,500 pixels. My algebra is a little weak,
but I believe that calculates to approximately 2167x1300.
Now turn to print size. With the original image (2500x1500) you could get a
print at 300 ppi of 8.33 inches by 5 inches (2500/300, 1500/300); with the
smaller number of the pixels in the cropped version, your 300 ppi print
would now be 7.22 inches by 4.33 inches (2167/300, 1300/300). Or, if you
still wanted a print of 8.33 x 5, your ppi would be reduced to 260 ppi.
All this is way more detail than you really need to go through to get the
product you're after: clear images, 4x5 size, 4 to a page. If your
starting 4x5's are 300 ppi or greater, setting the Picture Package
resolution to 300 is okay, 'cause it'll resize your 4x5's, throwing away the
extra pixels that don't add anything to the printed picture quality.
Whew! Enough math for a Sunday!!