CG and performance
Every airplane will balance out differently, but ideally, you'd want the empty CG to be at the forward limit, as fuel, people and baggage move it aft.
One of my flying buds built a 4 with a Hartzel CS prop and battery in the baggage compartment. He's running an O-360 and has an empty CG that's right on the forward limit.....good stuff!
My RV-4 has an O-360, with Sensenich fixed pitch metal prop and I have a gel cell battery in the foward center console. That should put the CG close to the foward limit, but with a lot of paint and a little extra metal in the rear fuselage, I have an empty CG that's about 4" aft of the fwd limit...not the best.
That said, I can still take a 200# passenger, but the elevator stick force becomes very light. If you fly with a CG aft of the limit, the elevator will have trouble trying to decide if it wants downforce or upforce.
The 4 is the most nimble two seat RV, but when you add a 200# passenger, the takeoff and climb performance really takes a hit. Also, with a rear seat passenger and associated aft CG, the airplane lands differently and even a slight sink rate at touch down when making a wheels landing will create the old tail dragger "jounce." This is even worse in an airplane like mine that already has an aft CG, so smooth is the word. Wheel landings are easiest with a big guy in the back. When solo, three point landings are pretty easy.
That said, I've put 1,000 hrs on the machine, including a lot of aggressive flying like acro/tailchase/dogfighting and back country flying. It's flown great and I've loved every minute, but I still wish I had more of a foward CG.
Haller Airpark, FL 7FL4