Similar to the first gen model, the LCD screen is factory glued to the digitizer, while on the 3G and 3GS, the two can be separated. This really only matters to repair people who will have to raise prices to fix the screen as two parts will have to be replaced at the same time. The front panel on the 4G model is 1/4 inch longer than the earlier models. Above the speaker is a mirror like surface that could be the proximity sensor. If it is the sensor, it would be in a different location than the other iPhone models.
Taking the leap of faith and believing that these are the real parts for the 4th gen iPhone model, it would appear that Apple is sticking with 'tried and true' as opposed to really shaking things up. Of course, this is only part of the story and there could be other hardware changes to come and certainly we would expect to see some changes in the software.
Apple iPhone 3GS Specifications | Review