Yes, you can do that.
However, if you're flying an RV-4 with a wood prop, short gear legs, and an O-320... you're going to make it even more tail heavy due to the extra weight (counterbalance, etc.) of the RV-8 tail.
If you're flying an RV-4 with a metal prop, etc.... maybe.
The point is that you need to check your W&B before doing anything.
A complete RV-4 tail weighs about 45# w/o paint. I don't have the RV-8 tail weight. Anyone?