Matt Laur , Jun 06, 2010; 04:31 p.m.
4 inches x 300 pixels per inch (dots per inch) = 1200 pixels.
5 inches x 300 ppi = 1500 pixels.
So, resize a copy of the image to 1200 x 1500 pixels. Every piece of photo editing software has some method for resizing, and all of them differ a bit in terms of how you enter the parameters for resizing. But by the time the dust settles, it's pixels. It's always pixels. X pixels wide, and Y pixels tall. If you deal in absolute pixels, you'll always know what you've got.
Saying 4 x 5 inches at 300 ppi is just the same as saying as 1200 x 1500 pixels.
As for "dpi" vs. "ppi" ... for these sorts of discussions, they're interchangeable. The term "dpi" is also used in more hair-splitting conversations about how ink is deposited during printing, etc., but don't worry about that part.