The distance from a plane (a^t)x = c (in m-dimensional space) to the origin is
|c| / ||a||.
How far is the plane x1+x2-x3-x4=8 from the origin, and what point on it is nearest?
What are a and c? Apply your formula to them.
Another way to do this is to note that the vector <1, 1, -1, -1> is perpendicular to the plane and so the point on the plane closest to the origin lies on the line through the origin having that direction vector. Write parametric equations for the line and determine where it crosses the plane.