4 million over how many years? We replaced free movement for europeans (a lot of whom would stay a few years then go home) with visa's for anyone from anywhere in the world who could earn just over minimum wage. Europeans mostly came as young single people - people from other continents want to bring their families with them - this adds pressure on houseing and education.
Before brexit net migration was 200,000, now its nearly a million - so yeah, it was a great idea.
As for Corbyn, he was as wrong about brexit as he is about pretty much everything else, as are you, chumpy.