What hemoglobin levels can lead to coma or death?

Depends how you get to that level and there are no rules. If someone is slowly dropping their hemoglobin, they can tolerate 3 & 4, as you did. I have seen many people in the ER with levels that low-they may not feel great but they are awake & talking. If you are bleeding out rapidly, then obviously a low level will result in death fast. Coma is usually not one symptom of low hemoglobin. I would not really quote a causal relationship between Hgb and death/level of consciousness because it is not established in absolute numbers.