Steiner tree four points example
You are asking the question for Euclidean Steiner Trees. For Euclidean Steiner Tree problem one can show that "points added to the graph (Steiner points) must have a degree of three, and the three edges incident to such a point must form three 120 degree angles". That's why you need two steiner vertices instead of just one for 4 point solution. If you take just one extra point the degree will be 4 and angles also will not remain 120 degree.

To prove the property of Euclidean Steiner Trees take help of your favorite search engine. A good reference. See section 3.