4 square feet per adult standard-sized hen is the recommended *minimum*.
The numbers that manufacturers of prefab coops claim they can hold are based on the legal minimums for commercially-kept birds -- birds that are often debeaked to prevent aggressive behavior from doing damage and which are kept in an intensively-managed system with artificial ventilation where they will be culled by the time they're 2-3 years old.
While it is true that there are no hard-and-fast RULES when it comes to living animals, people here at BYC have seen many, many people come to seek advice for health, behavioral, and/or sanitation problems that have boiled down to not giving their chickens adequate space and/or adequate ventilation.