I must say I'm not a fan of the idea.

Having content for groups of 4 is great news, and I welcome it for the same reasons as you. But I do appreciate that the endgame content is well categorized; and in my opinion, it's normal to have instances that are only accessible to certain people. Here, those who make the effort to set up and participate in fixed roosters, which requires a certain investment.

And it is out of respect for this investment, which people like you and me are not ready to make (< absolutely not a problem, just an observation) that we should keep these instances reserved for groups of 8.

The other concern I see with the idea is that one of the main difficulties of savages is coordination. Many groups don't get clean not because the individual player doesn't know the mechanics, but because the group doesn't work together. And this "coordination" aspect is strongly lost in groups of 4, since it is a priori easier to function with 3 other players than with 7 others. Minimizing the importance of coordination in high-level instances would be the equivalent of denaturing them in my opinion.