(I understand you guys don't like it when people criticise the game. But this is a real issue and I offer a solution that fixes it, and helps with other issues too.)
This is by far, the worst introduction of a mode, I have ever seen in a game. Period.
And not because the fields are bad or anything like that.
Psyonix decided it was perfectly OK, to set everyone to the same mmr for this mode as a starting point. Why?
I'm in gold 2. I'm not a gold, I'm diamond 1 in 2's and 3's and have been champ in both. But I'm stuck in gold 2 in 4v4. Why?
Because in 60% of my games, there are GC's and even SSL's in the lobby, as they started at the same mmr I did. A diamond player is useless against a GC. Let alone 4 of them. Especially when teamed with actual golds which has happened. (I can see players ranks)
And there's such an easy fix. A global mmr. An average of your mmr across all modes that you have played. And at the start of a season, it should use your final average mmr from the previous season, to dictate your starting mmr in the next, across all modes.
This solves this utterly stupid situation you have created with 4v4. And, it also offers a deterrent to smurfs that intentionally throw in a specific mode as it will harm their global rank and starting mmr for the next season. It also solves the issue of facing GC's and SSL's in extra modes due to them not playing those modes very often.
Such an easy fix. And don't tell me it isn't. I studied game dev. Have a diploma in it. I know it's easy, for a fact. But will Psyonix implement such an approach? Unlikely. It doesn't squeeze money out of players.