Does 4 parts per million of anthropogenically generated CO2 really have the impact suggested?

First justify the claim. The figure being banded about is a doubling of CO₂, compared to pre-industrial levels. How near are we currently? Very!
Pre-industrial life sustained globally for at least 2 million years of primates burning trees to cook food, shows climate change indistinguishable from natural changes caused by (take yer pick): magma release/ash, earth's orbit variations, solar flares and (other).

4 ppm refers to IQ IMNSHO