Earlier quoted context omitted.
Apple doesn't need to buy the Big 4, they're quickly disrupting them. The Big 4 used to offer, publicity, promotion, and distribution--now iTunes does all of that. Record labels are dinosaurs who are unwilling or incapable of adapting to survive.
Apple is not competing with the record labels, so they can't disrupt them. Labels are not going anywhere, artists need labels for various reasons. The function of labels is changing which is what is interesting. They labels are hesitant to change their model.
But you're right, Apple isn't trying to be a one stop shop like a record label, Apple is only going after a piece--that is what record labels should be adjusting to. Record labels should try to differentiate and offer whatever services Apple isn't--marketing, advertising, studio space, etc.