I'm combining nembutal with a body bag used for cadavers because placing a bag over my head makes me feel claustrophobic. But I think it might take hours for enough co2 build up to kill me in a body bag. so will 4grams be enough to keep me knocked out til I die?
It depends on whether you mean 4g of the liquid or 4g of nembutal as in its active ingredient (that would be over half of the bottle of N).
I don't have any medical knowledge, but in my opinion, if 6.3g (100ml) is enough to kill a person, 4g would definitely knock you out for long enough.
To quote an article "Death commonly occurs after 2 to 10 grams of ingested barbiturate."