Not only that, but +4℃ is also when megafauna - any species over 45Kg (100lbs) - start to go extinct.

And that includes humans.

At that point, we’re looking at our global population free-fall cratering into the 1-2B range over 1-3 decades, at best. Worst-case is under 500M, with a full reversion back to the Bronze Age or earlier. And a continuing persistent (slower) decline down to a big fat zero.

Wouldn't you have to subtract out the warming to date and add that difference rather than 5 on top?