Jamie,
I have inserted pictures...hopefully they will be clear enough.
This is an overlay of the RV-3 horizontal tail (green) and the RV-4 horizontal (white). Not telling you anything you don't already know...but illustrating the size difference. The RV-4 tail will put much more downforce on the tail, which will have a major effect on the handling of the 3.
Here is an overlay of the RV-3 vertical (blue) and RV-4 vertical (white). Again the 4 tail is larger, which you already knew. I imagine the 4 vertical will increase the yaw stability, but will require modifications to the fuselage structure to attach. The attachment points will need to be moved well forward of the RV-3 position, and the fuselage will need to be modified accordingly.
As a final thought...remember that the RV-4 empenage is not matched hole. It may be pre-punched, but you will still be doing a lot of pilot hole drilling and fitting.
Overall, I would not recommend putting the 4 tail on a 3. The increased downforce from the horizontal will have an adverse effect on the handling. To keep the handling the same, you would need to move the larger tail forward, and that is a major structural change you don't want to deal with...it would be
much less work to just build the original 3 tail!
Warmest regards,