The rumor floating around today about the iPhone is that it will use a thinner display technology made by Sharp. That could possibly mean better battery life for you. And it could mean a 4G LTE iPhone would be able to make it through the day without several charges.
While WSJ doesn't mention the screen by name, it could be that the tech that's being talked about is the IGZO tech that didn't make it into the new iPad. IGZO is basically smaller transistors that allow light to pass through the display, which means, you'll need fewer LEDs for the same amount of screen brightness, which in turn means you'll only need lower power consumption to operate it.
And if that's the case, and if Sharp's IGZO screen is being used, then there's possibly extra space from a thinner screen would almost certainly be filled by more battery, and the lower power consumption from the IGZO would compound the gains. The drawback is that it'll probably heat up more than your iPhone 4S now.