I think, for 4.0 to work well with pre-established patterns that 2.0/3.0 use, the content has to be semi-gated. What I mean is Classes/Races/Zones should be made available to access just by purchasing the expansion, but MSQ/Dungeons should require at least 2.55 MSQ to be completed, if not 3.X as well.
I feel this would work best because we'd stop people from going "I just want to play as DRK now while I'm just unlocking Brayflox!" while still having them play through the old dungeons/content to learn mechanics. Let everyone play as Red Mage (or Blue...or Dancer...or whatever else they're teasing will be next xP) instantly. If anything it'll help them learn the class faster, and decide quicker if they want to really use it.
I feel this would work best because we'd stop people from going "I just want to play as DRK now while I'm just unlocking Brayflox!" while still having them play through the old dungeons/content to learn mechanics. Let everyone play as Red Mage (or Blue...or Dancer...or whatever else they're teasing will be next xP) instantly. If anything it'll help them learn the class faster, and decide quicker if they want to really use it.