I went out an measured some airplanes this weekend.
I had access to a -6A, -8 and 9A and I have plans for the -4.
This is what I found:
The 9A is totally different than the others, it has a "hershey bar" shaped H-stab. So, I only compared measurements of the -4, -6, and -8.
The H-stab on the -6A and the -8 seem identical. the elevators also measure the same. On the -4, the only difference is the tip of the H-stab is a little narrower, but the root is the same and the length is the same. This results in the -4 having a little more sweep back to the leading edge. So that would mean a little less surface area.
On the rudder, the -4 and the 6 measure about the same. On the -8 rudder, the bottom is the same width but it is taller. it also has the counter weight on the top. It looks like they just extended the top enough to include the counter weight.
Again on the V-stab. the -4 and the -6 look the same. The -8 is has the same measurement on the bottom of the V-stab, but it is taller.
Keep in mind i just did quick measurements with a tape measure. it would be interesting to compare the distance between the actual ribs on the -8 plans.
If someone has a set of -8 plans, and they can send me some of those dimensions, I will compare them to the -4 plans.
Without more details, I would say in general, if you put an -8 tail on a -4, you would have about the same surface area on the elevator and a little more on the H-stab. If you mod the -8 rudder and V-stab to leave off the counter weight, then you would end up with about the same size V-stab and rudder as a standard -4 and you would have pretty close to the same weight since you removed the counter weight.