I checked the history of the 4 man leaderboards. I wanted to summarizes and give highlights of the available information. As we know, greater rift clears of 4 man groups are significantly higher than solo players. As such, inclusion in the 4 man meta greatly increases that classes ability to get higher level legendary gems as well as XP/paragon levels.
Please find below the classes and seasons which they are part of the 4 man meta.
Barbarian
4,5,6,7,8,9,10,11,12,13,14,15,16,17
Crusader
1,2,3,4
Demon Hunter
1,1,2,2
Monk
4,4,5,6,7,7,8,8,9,9,10,11,12,13,14,15,16,17
Necromancer (Introduced Season 11)
11,12,13,14,15,16,17
Witch Doctor
1,2,3,5,6,7,8,9,10,11
Wizard
3,3,10,12,13,14,15,16,17
Since season 4, two classes (barbarians and monks) are always part of the 4 man meta.
No crusader or demon hunter has been part of the four man meta after season 4.
The four man meta in terms of class composition has not changed since season 12 where it is Barbarian, Monk, Necromancer and Wizard.
I think these observation suggest that there needs to be a significant overhaul to encourage greater class diversity in the 4 man meta.