I'm not sure it's 4X the lethal dose. I think that's Chinese whispers. It was 4X the dose known to at minimum present risk of suicide. I haven't seen the original statement only that he was under the influence at the time.

It's hard to OD on opiates that I'm aware of in in the same way you would other poisons though fentanyl is particularly potent. He may have taken a high dose as his body weight was high.

He was on a cocktail of drugs though. Speed and weed as well.