4 degrees is the apocalyptic scenario. The vast majority of oxygen in the atmosphere is provided not by trees or any plants, but by the algae and cyanobacteria in the ocean. At the 4 degree threshold, they can't do aerobic respiration anymore, so they switch to anaerobic respiration. This means they stop producing oxygen, drastically reducing the amount of oxygen in the atmosphere and drastically increasing the carbon dioxide. This does two things: kills any large fauna, humans included, and the additional carbon dioxide continues to act as a greenhouse gas, accelerating the effect even further. Eventually, after almost all oxygen breathing life is dead, we reach equilibrium, assuming your definition of 'we" includes insects, because that's basically all that would be left. If there's a risk of reaching the 4 degree threshold, we would be forced into taking our chances with the literal nuclear option of deliberately inducing a nuclear winter.