The -8 added a counterbalance to the rudder (missing on the -4 & -6); the -7 originally used the -8 rudder but subsequent spin testing on the wider span -7 left them 'uncomfortable' with spin recovery, so they started shipping the larger -9 rudder with -7 kits.I think the tail on the 7 was more than just reduced inventory. The 6”s vertical has always not been not as effective as the same on the 4. The wider fuselage needed more vertical. That the 6 exploded the RV world was the magic combination. They were not going to not miss out and not have an improved side by side. The 4 and 8 are the best dream makers; the 6 and 7 are the best reality makers.
When you say 'vertical', do you mean the stabilizer? I owned -4s for about 25 years, and I'm about 2 years into owning a -6. Never thought to measure the -4 & compare it to the -6, but I haven't noticed any significant differences between the -4 & -6; not much more variation than between the two -4s I owned. But I might not be tuned up enough as a pilot to detect the issues.