Just because the battery life is longer doesn't mean that it is all from energy savings, some of it comes from the battery. But, I figured I'd do an actual synthetic test against my power meter that I have on a battery box I use for camping. I was actually pretty surprised by the results.
So the 3GS draws around 2.7 to 3 watts while fully charged and connected to the battery box. This assumes max display brightness, wifi and bluetooth on, music playing, and TomTom running a demo route. The draw that the USB port allows for is 2.5 watts (half an amp at 5V).
I hooked up my iPhone 4 in the same way and let it charge to full. For the first 15 seconds or so I saw it hover at about 2.5 watts once I confirmed it was full. Seems there is some lag time where it'll draw enough power in order to try to trickle charge any small drain on the battery. After that it dropped to a mere 1.7 to 2 watts and held there for a good 4 minutes. It dropped even lower when messing with the menus rather than having TomTom render the map display. I used Pandora in this case over WiFi just to see how network access would affect the battery draw. It is still likely real situations will draw a bit more power, but half a watt is a decent enough buffer.
So yes, the iPhone 4 will charge on the tomtom car kit dock. Although fairly slowly.
That said, the main benefits of the car kit for the iPhone 4 is the charging and hands free features. The GPS chip and antenna in the iPhone 4 is on par, and in a couple ways superior to what is in the car kit.