4vs4 is no more hardcore or casual then 1vs1 is just a different flavor.
It is definitely more casual. I'm sorry, that's beyond just an opinion, it just is.
1. I would've said this in 2v2 but the higher you go up in team games it just gets more true but the fact you can get carried already puts it beyond 1v1. In 1v1 you are responsible for everything, in 2v2 and up your mistakes can get covered. I have both carried and been carried and acknowledge that I may have done nothing the whole game and still won. There are games where the other guy could've played with an AI and still won.
2. You can do things you wouldn't get away with in 1v1. (e.g. starting with 2x snipers cause your friend went 4 riflemen and 3 rear echelon, doing bloody 3xsoviet medical house while the brit laughs and gets Churchill Croc + AT Gun Line ready)
3. The sheer difference in player knowledge and skill. I rarely fight guys anymore in 1v1 that don't know how to retreat. (Up to about top 800 and thats how it should be) Up into the top 500's of 2v2 I would still run into players that did not know how to retreat, could only do one build order (e.g. 3 grens, 1 MG, 1 Pz4, 1 Tiger and NEVER DEVIATE), didn't know how the game worked. (omg, my AT Snare didn't damage your full health tank!) This point is anecdotal but I'd like to hear if anyone heard of a 2v2+ player who put it more work and demonstrated more knowledge than a dedicated 1v1 player. The amount of times I've seen a 2v2 player however quit because his early cheese didn't work (3+ scout car flamer rush. That was amusing) shows a worrying lack of ability to adapt the higher up the team games you go.
As a final point, yes, in an ideal world all modes would be balanced for play within each other. But Relic has always made competitive games with a focus on balancing the 1v1 so that in a 1v1 everyone has a 50% chance of winning at game outset. It just so happens that the way they make their games makes it very difficult to balance from 1v1 up (where timings and approximate resource level can be predicted) Their balance in team games has gotten much better but could still use some work. However if dedicated team players want to show that their balance issues should take precedence over 1v1 I think a well though out and persuasive argument is needed beyond simply 'It is the popular!' (I could give less of a shit, Call of Duty is popular but if you said you were num 1 in the world I wouldn't care.) or 'It would die!' (I played CoH for around 6-7 years, well into and beyond its death throes and had a blast. I even paid money to be in the beta release) I'd rather play a good game with a dwindling player base than play a dumbed down game with a healthier (relatively speaking...) player base.