4vs4 winrates were never depending on balance but only on the playerbase. The playerbase with most players has more wins because the teams have a better elo. While the team with the smaller playerbase has a more diverse (worse) elo.
When brits came out, everybody played brits, since allie winrates were much higher. Now more people play axis in 4vs4 and we have again a higher axis winrate.
Thats random 4vs4. 4vs4 AT has probably a 90 - 100 % winrate no matter what
Player base willing to search in one side also depend on balance. Someone, probably a team found that building an army around early 2x ISG/Mortar (I agree mortar are fine) is more efficient than anything else and then everyone started to do that as Axis.
From there, 3 options:
1- Do the same as allied: failed, pak how are ususally too late and force you to play capt tier, Brit's mortar pit are not efficient at all vs ISG and Sov is the only faction having early mew mortar + Cp2 heavy ones. Honestly, and I'm perfectly fine with the fact Allied can't match them: but since its too good and drive the game in a Verdun style gameplay, I simply don't like it.
2- Switch Axis for the patch time being until better a better meta raise
3- Stop playing 4vs4 until next patch and a better meta
2 of the 3 options impact 100% the player base search rate.