156/147 differences

I was just curios to find out what are the differences between the 147 GTA and 156 GTA, besides the obvious. I am reffering to the engine. As far as I know, the engine is the same, 100% identical, and the cars were both launched in 2002, so they are both Euro 3 compliant.
But I hear some people saying the 147 GTA is Euro 4, which I find strange.

Also, is there any difference between the pre-2004 and post-2004 GTAs, except for the larger brakes ?