A nerd reply without math is if you were to slice the tank on a meat slicer (into thin slices), each slice would be supported directly below it by the stand. This is exactly how an evenly supported tank is distributed. When you support the tank by the four corners, the glass must bear the stress, strain, and deformation. The elastic modulus of glass is very low, meaning that it deforms very little before failing.
The answer for the center support is if there are three sets of supports, the centers will carry half of the load (or a third, I am not sure but I am working it out on a sleep deprived brain. I'd go with half.)
I do not do math while sleep deprived but the solution is solved using moment arms. The equation is M=Fd. M is the moment, F is the force, d is the distance from the moment. The equation of equilibrium is (sum)Fx=0 or add all of the moments together letting them equal zero. If your center support is represented by one variable and the outsides by a different variable, then the centers can be solved in terms of the outside supports.
Another way is using a mind exercise. Imagine cutting the tank in half, resting the two halves on three sets of supports. The center supports will hold twice the weight as each of the outside supports. (each half will be supported evenly by an outside set of supports and the centers)