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Emissions are generated by quantity of fuel burned and the efficiency ratio (lean or rich of stoic) at which said quantity of fuel is burned.
Efficiency is a measure of how much power is extracted from the fuel burned and is affected by timing, compression ratio, efficiency ratio, frictional losses, rotating mass and Cd. (Perhaps I've missed something?)
The new 4s run lower compression pistons and are therefore less thermally efficient in that area. Perhaps the new car can run leaner ER than the old one. The new 4s have less reciprocating mass; so gain some efficiency there.
I would bet that at full song, the old 6 would compare favorably to the new 2L 4 in terms of efficiency.
But then Porsche does not car so long as it performs well under the guidelines of the regs.
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