V2 had only changes to the fuel curve.
"In the Stage One code we retained the shape of the Subaru fuel map while moving the entire fuel curve down to a focus around 12.9 to 1."
www.ecutune.com
But V4 had changes to the fuel and timing curves.
"Starting in Stage I v4 performance was further increased by adding ignition advance to a table used in conjunction with the dual knock sensors to actively seek the borderline of knock."
www.ecutune.com
I had V2 and then went to V4. I felt a diffence, but it was not as much as going from OEM to V2. I would say that V4 on the first day felt like V2 with a week of learn time.