combinatorical acquaintanceship problem Group of people went hiking.
It's given that if we pick any four of them, than at least one knows everybody in that quad.
 We have to prove, than in group everybody knows everybody, except at most 3 persons.
I tried to sketch a problem for the case when group  size is 5, to get some idea of how it might be possible to prove. Statement hold, but I really don't know way to prove it.
I would appreciate some insights, thanks.

Hint:
You cannot have four distinct people with "$A$ and $B$ do not know each other" and "$C$ and $D$ do not know each other", as it contradicts "if we pick any four of them, than at least one knows everybody in that quad".
