If you don’t have 2/4 piece by Tuesday

[Dartakan-ragnaros (Dartakan)]

To get a tier piece it needs to have a corresponding piece, if it doesn’t have one you wont create a tier piece. It does sound a bit ambiguous, but they clarify in the next line:

@Linxy

Note that the 2-piece and 4-piece set bonuses will only be on the helms, shoulders, chests, gloves, and legs.

Is 100% clear that you need to use a shoulder, chest, gloves and legs to get a tier set. Again, is not like the vast majority of the player base even read that post, what matters is that the in game interface is clear on what you are creating and it certainly is.