Dude, in all due respect, I think you might be thinking too much about the subject.
Far as I'm concerned, the reason Disney hasn't realeased either seasons is simply because they just don't want to. Simple as that.
If they could simply edit out songs that couldn't be cleared for the first three, then they would have done that by now.
Also, considering Disney has enough money to find a cure for cancer, I doubt music rights or estates are an issue. I think in Disney's mind, they just don't want to have to spend their precious money on songs knowing them, which is why the first three were edited to begin with.