I play exclusively 4v4…
Every game… and i mean every game… if the teams are actually even or close to even… which is hardly ever… 9 out of 10 times my team will win.
The only time my team is losing is when my team is seriously out-ranked by at least 5+ ranks above. E.g. team of plat 1s and 2s vs. a team of two diamonds 2/3s and plat 2/3s… which happens probably 4 out of 5 matches.
VERY RARELY am I ever on the team that out-ranks the opponent team.
Why is this?
There’s 3 things I can think of that would give this result:
All players have a ‘hidden’ MMR. This is a rating carried between ranked and quick match, and your hidden rating is actually higher than your ranked rating. This gives you opponents that challenge you more than the same rank. I get this one a lot since I mainly play quickmatch, climbing mid season as a plat vs conquerors @_@
There’s an imbalance in cumulative MMR. The game adds a teams MMR together and places them against a team of the same rating. For example, (1300, 1000, 1500, 800 vs 1150, 1150, 1150, 1150). Both teams have the same cumulative MMR, 4600, but one team gets a conq 2 player and a dia 2 player, vs all platinum.
This is less common, but still possible. You or your opponents have queued for several minutes, this causes higher ranked players to get matched with lower ranked players.
And every 1v1 you lose, your opponent is trash. Jokes aside, you should try your luck at 1v1. Why are you playing teams with randoms that drag you down, I don’t get it? Either that or find some team mates you can coordinate with.
I dunno I just enjoy team games instead of 1on1. shrug
You don’t seem to enjoy them though. 
So I would start by asking myself why do I want to play them and get to the bottom of it.