I've ridden the 12/10 and 8M now. The V4 is the best Reo yet for sure. For me the differences are subtle but definitely noticeable, the kite turns better with better power delivery and control in the turn, drifts better, and is more stable.
I've taken some pics of an 8m V3 and 8M V4 side by side, and you can visibly see the kites differences.
Struts, this is the centre end to end. All 3 struts are thicker and a different shape, so they are more rigid and support the canopy much better.
Dacron in the tips minimised, saves weight and makes the kite more responsive
Leading edge is a different shape. You can see the increased number of wear patches, there is one on every segment seam now.
Tip shape is different and the tip bridle point is moved significantly to the outside of the kite, I think this is where the difference in turn I can feel is coming from.
segment seam protection
Shows the difference in shape
Difference in tip shape and size too, new V5 is on the outside (Right side)
Tip shape difference
Trailing edge battens, more of them and longer/wider
TE battens closeup
One laid on top of the other, does not show the differences nearly as well as I thought it might, but included for those that like to see this shot.