Certainly you're familiar with The Population Bomb from 1968[1], predicting mass starvation by the '80s.

Africa is huge, and 4 billion could live there. But will there be mass starvation? If they got to 4 billion, then I'd say not ;-)

[1] https://en.wikipedia.org/wiki/The_Population_Bomb

Only if climate change doesn't render Malthus redundant by turning the planet unlivable well before the end of the century as it looks bound to happen. You at least won't be be able to put the blame for that on africans... Also current refugee crisis in Europe is caused to a large extent by people fleeing political crisis in the middle east, a significantly wealthier region than Africa. The trickle of african migrants that was reaching Europe illegally via the mediterranean has always been overly exaggerated numbers-wise. It's a sad reality for those who see no option but to risk their lives that way, but I doubt Europe will ever have anything to fear from boat immigrants from Africa like you seem to suggest.

> It is however likely that Malthus will intervene

Malthus isn't an inevitability. Strong institutions that balance local-responsiveness and long-term-planning could have prevented things like the Irish Potato Famine.