They recently made Shadow lair mobs scale damage/defense after level 30. So there are some hints that they would be making a Tier 4 and maybe missions along with it. But those would probably take eons to develop (I mean, just look at how long it took them to churn out treas(h)ure vault). But I highly doubt it will be worth the grind seeing as GH seems to think punishing difficulty and low reward is what the community (maybe the masochists) want.