Phase 2 release of tier 5 will break up the TBC experience. Tier 4 does not stand up on it's own. TBC is defined by the long winding road that was the raiding and attunement journey. It would be better to release TBC classic later and have all launch content available as it was in January 2007.
And if anyone thinks this would obsolete tier 4, you do not understand how BC worked. Content pacing is a concept for the modern game. BC content innately paces itself.