I come at the debate as a skeptic - that is skeptical that we face any real problem from rising CO2 levels.
But, I don't think the 0.04 % statistic is relevant. What's important is density, because that determines the probability that a light photon will collide with a molecule* as it travels. "395 parts per million" doesn't tell us anything about density, it just tells us the ratio of CO2 to the rest.
A figure that I just looked up -- which I find surprising -- is that 1 cubic metre of air at sea level has a mass of 1.225 kg (http://en.wikipedia.org/wiki/Density_of_air). So there's actually quite a lot of molecules floating around in air, of which CO2 is a small 0.04 % of. It's a bit like a spaceship flying blind through an asteroid field, the more asteroids the greater chance of hitting one; doesn't matter what ratio the asteroids constitute with regard to other stuff that doesn't damage the spaceship.
*Only some molecules will cause interference (greenhouse gases) and depends on wavelength of photon.
