exactly nine planes!
* * * * *
I would have said 4 - corresponding to the four faces of a tetrahedron. Of course, non-collinear does not mean non-coplanar so all four points could be in the same single plane!
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I dont think that "If four points are collinear they are also coplaner," is the same thing as "If four points are coplaner they are also collinear,". The definition of collinear is at least three points on the same line. To define a plane is to have threenoncollinear points.
Any 4 points can lie in a plane, 3 points determine a plane and just take the 4th to be say the origin.
Sometimes.
The shortest distance between four points is a straight line to and from each individual point. If all four points are aligned, the result will be a single straight line through all four points.
Are the four quadrants.