not to mention that the iphone 3G/3GS does not have the drivers for the screen of the iPhone 4
the software for a certain iPhone is made to work only with that iPhone, and only with the hardware that iPhone has
all the graphics are meant for the lower resolution screen, and the phone wouldn't recognize all those extra pixels, as its programmed to think that its screen is the only possible screen (hooking up to external screens is obviously different), and so it wouldn't know what to do with the new screen
that is, if it is even able to display an image on the new screen at all