why did SE restrict FC house purchases to FC's with four people

Because at that point it might as well be a personal.

i can craft. i can have friends or random temporally join fc to craft

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Originally Posted by KoroushSensei

i can craft. i can have friends or random temporally join fc to craft

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...and you can have friends join the FC temporaly to buy a house, too, right?

Several FC-related contents require a minimum of 4 people. Founding the FC requires a minimum of 4 people. Makes sense for them to say "We assume that an FC has at least 4 people in it" and make that a requirement for any FC-content, like... housing, as long as its restricted to FC-only.