Earth Temperature Timeline

so.. how cold does it have to get to get the glaciers back? Does anyone do climate simulations of what would happen if the earth cooled 4 degrees C?

The Last Glacial Maximum 20,000 years ago was about 5.8 °C cooler relative to present day temperatures. But it's not really just the temperatures, it's how fast they change and what causes them to change. This is why geoengineering is not a trivial solution to climate change.