3v is still a single over head cam. The 4v is a dual over head cam. So the heads are much larger and have 4 cams instead of just 2. You can use a 3v block and put 4v heads on it, as long as you put 4v everything else along with the ECU.
Faster than a speeding...............
2001 Bullitt being prepped for track duty
Corners > Straight line