The ff have had more the four members many times. Even wizard talked about how they are called four but have had many more in a issue. Ms marvel was the 5th member for a time and before heroes reborn they had about 7 members!
Still don't get the "spidey can't join a team" talk. Spidey had been on more teams iron man or cap! Even before bendis spidey was a FULL avengers for three issues before going in as a reserve. That's a longer time then the hulk!
Also don't forget this what if story. (didn't have a happy ending however)
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