We should prepare structure for this, for Brett Lawson to plug content into.
1) 3->4 migration. This will be a short section on the migration page with just exceptions found by Brett, QE, and our SDK Docs testing.
Brett will plug content into this, for Docs to revise.
2) 2->4 migration. (kinda - we're now keeping it in the same page, so it's naturally going to be a 2->4 content. Depending on what Brett's list consists of, we may refer to 4.0 earlier too.)