The RV-4 is a delight on short farm strips and its undercarriage is much less stiff and bouncy than the RV-8’s, making short landings easier in the -4. They both can and do operate from very small strips and can go to places where you would normally see Super Cubs.
The 180 hp RV-4 is faster and lighter than the 200 hp RV-8 (both with c/s props), this is from practical experience.
The RV-8 has a bigger fuselage and is wider than the 4’s - useful for very large pilots! There is a bit more room for the passenger as well. The room for luggage behind the seats is almost the same, but the 8 has another small locker behind the firewall. The -8 is available as a quick build kit, the -4 is a normal kit.
The big difference: only the -4 is cleared for aerobatics in UK +6g/-3g, and these aircraft roll and loop beautifully.