It is the same hardware
Same SoC! All have 128MB Ram!
Bzzzt... Nope. Myth. Not the same hardware.
The 2 phones (1st gen and 3G) have the same application processor. But these phones are really small multi-processor systems. There are thought to be 3 to 5 processors inside each device (and more than one ARM CPU in each, according to EETimes). OS 4.x probably requires and includes significant changes for more than one of those processors (baseband CPU most likely) which are not identical between the two generation devices (different performance, different embedded memory size, different IO registers, etc.)