I'll assume Jean is not included unless specified by OP. I'll also assume that the teams have basic knowledge of each other.
That said, giving the teams 4 days of prep puts this solidly in the Avengers' favor. The Avengers have Tony, Steve and Banner to plan for fight. The Brotherhood has... Magneto?
All the Avengers need to do is make sure no one wears anything metal. So make sure BW and Hawkeye use plastic or carbon fiber guns and bows. Make IM a carbon fiber suit, or maybe Tony can figure out a way to de-magnetize his armor. Make Cap leave his shield. Might even leave Vision. Tell Thor not to wear armor (though Mjolnir should be powerful enough to resist Magneto's powers).
Then on the battle itself that starts half a mile apart, just get Thor to spam the Brotherhood with lightning. Even if Magneto surrounds them with metal barriers they will probably still fry. That only leaves Juggs standing and he has to face the entire Avengers team.
Now if it was just a random encounter, things will be a lot more interesting.
I agree prep pretty much hands the Avengers the win due to entire team being either genius, assasin,spies,supersoldiers and thousand year old war gods.
But Mags...Juggs is useless. Either Thor, Hulk and maybe even Vision could bfr his ass in one hit. Hell, Iron Man has better strength feats than juggs.
Im about to make a mags gauntlet because I think he can decimate the team if they arent careful. He will drop a damn bridge or city block on their heads and it will be goodnight for majority of team.
Also, don't think demagnetizing Tony's suit would make a difference. Movie Magneto's powers aren't quite like his comic powers, despite the name. His powers are described in both X3 and DoFP as "control over metal", not magnetism. And he has displayed control over all kinds of different metals in the past as well.