code: public Vector4d add(double x, double y, double z, double w) {
          return add(x, y, z, w, thisOrNew());
      }
docs: Add <code>(x, y, z, w)</code> to this.
  
  @param x
  the x component to add
  @param y
  the y component to add
  @param z
  the z component to add
  @param w
  the w component to add
  @return a vector holding the result