You can't make a 4 inch HD screen! You know how small everything would be!? It would make no sense!
Take your PC monitor and turn it up to max resolution...notice how all of the icons on your desktop and text get super tiny? THAT is the reason it isn't HD. That's all that HD refers to...pixels.
You guys that are whining about not having an HD screen are falling for some marketing mumbo jumbo and not really thinking about what it means.