The main differences between Euro 2 and Euro 4 emissions standards are as follows:

For petrol vehicles, Euro 2 set limits at 2.2 g/km for CO and 0.5 g/km for HC+NOx, while Euro 4 reduced these limits to 1.0 g/km for CO, 0.10 g/km for HC, and 0.08 g/km for NOx.

For diesel vehicles, Euro 2 had limits of 1.0 g/km for CO, 0.7 g/km for HC+NOx, and 0.08 g/km for PM. In contrast, Euro 4 tightened these to 0.50 g/km for CO, 0.30 g/km for HC+NOx, 0.25 g/km for NOx alone, and 0.025 g/km for PM.

Euro 4 also introduced the use of particulate filters in some diesel cars, which was not a requirement under Euro 2.