Question 4
If $A, B, C, D$ are four points in space, such that
$$
\angle A B C=\angle B C D=\angle C D A=\angle D A B=\pi / 2
$$
prove that $A, B, C, D$ lie in a plane.
$$
\angle A B C=\angle B C D=\angle C D A=\angle D A B=\pi / 2
$$
prove that $A, B, C, D$ lie in a plane.