What's the best way to take the human population from 7 Billion back down to around 4 billion?

Voluntary suicide. You first. Post when you've done it.

Maybe a few nuclear bombs, or some other devastating blow to humanity. We'll be okay with our current resources as long as we continue to educate others about sustainability and how to conserve the finite resources we have been consuming. Population in sustainability is one of the lesser subtopics that humans care to take action against (except China, but that's done with). Though, I would not be opposed to population restrictions internationally, it would be one hell of a controversial topic. Even then it would take centuries to steadily decline to 4 billion.

Give the nameless one a little more time.

Population is still going up, although slower, so is longevity. Accept that it's going to take at least a couple of centuries to decrease the population that much non-catastrophically and figure out how to sustain the overpopulation for that time span.

any real answer would be a pretty unpleasant one. unfortunately no countries in the world (that im aware of) are currently tackling this problem. we have a hard time accepting our rampant expansion and depletion of finite resources. getting the world to accept that there needs to be a reduction in human population before its really too late seems like nothing but a fantasy.

Make a worldwide "Hunger Games," but instead of representatives, just give them all a 1 week deadline. Whoever is still standing gets another year to live on the planet.