4-man variants for endgame content going forward?

[Darthgummibear]

but it will most likely end up having to deal with simpler team mechanics

This is really the key point right here. At level raid content is quite literally mechanically designed around the number of bodies on the field. You reduce the number of players, you reduce the complexity. If they were to do something like that they would basically have to create two completely different versions of the same raid, which would obviously lead to less content for everyone across the board.