Keep it OH so light. 320= good, Fixed wood prop=good. Keep the paint, interior and panel light but functional. For speed you might pick up a few MPH from a fast back deck. I like the bubble canopy look better but fast back is a little faster and should not add much weight.
The MOD that would be on my must list is a Sam James cowl or style (you could make your own if adventurous). If going fixed prop by all means get a long cowl and use a longer extension cowl.
Personally I'm a Sensenich metal prop fan. It out performs all fixed props in my opinion, and you have the option to re-pitch a metal prop. I say the Sensenich outperforms all others in my opinion. There's not much data or recent side-by-side comparisons of fixed props. I base my opinion on the fact metal props tend to out perform wood props in top speed (due to wood blades being thicker), and two blades tend to outperform three blades, especially for low HP engines. Low HP meaning less than 300-350 HP). Three blades have advantages, but top speed on lower HP engines is not one of those advantages. Van did a fixed prop fly-off/side-by comparison test a long time ago. The last prop fly-off Van did was for constant speed props but included one fixed Sensenich. The fixed Sensenich out performed several C/S props at 8,000ft @ 2,500 rpm.
In my opinion, between the Sensenich and Catto, the Sensenich is faster. However the Catto is lighter and likely smoother.
Be careful making your RV-4 too light on the nose, it will severely limit passenger and bags. Solo it's a dream to fly like this, with a light engine/prop set-up, but dual acro might be impossible except with all but the lightest passenger. Heavy paint (and primer) tends to move CG aft. On the other hand, if you have good basic forward CG (at empty weight) for dual ops, it tends to be a little nose heavy for solo ops. When I say nose heavy, I mean still within limits. You really only notice fwd CG when flying solo on approach. Also you have the option of putting a little ballast in the baggage for solo ops. Of course fwd CG is safer than too far aft CG out of limit, which is a no-go. Fwd Basic empty CG gives you more loading flexibly than aft. When trying to fly with passenger and bags you need nose weight. Everything you put in the plane +pilot, goes aft CG. If you end up nose light, with a tail heavy basic empty CG, you may NOT be able to do dual acro and passenger & bags will be limited for non-acro flying. Just keep this in mind. There are ways to play with CG, but choosing a super light or super heavy engine/prop combo makes your options limited. There are lots of 320/wood and 360-180HP (even 200HP)/constant speed prop combos in RV-4's. It's all about trade-offs. If you go with a light engine/prop and need more nose weight, you can always retain the heavy starter and alternator. Some use a harmonic weighted balancer on the hub between the engine and prop. This damper adds weight on the nose and claims to help increase the rotational inertia of light props for better engine idle.
If you don't go with a Sensenich, consider other brands of fixed wood two blade props, with out the fiberglass wrap. They are lighter and cheaper than Catto, with similar performance. The glass cover over wood is not impervious to rain or rock erosion. Catto makes good stuff, no doubt. I'm not saying Catto is bad, just look at all brands of props before you buy. I do like the composite wrap and integral crush plates of the Catto. However repair on a prop like that is a big question mark. The trick is don't damage it.
Wood props require you RE-torque prop bolts periodically because wood swells and shrinks with humidity. This is another advantage of a metal Sensenich, no need to re-torque and better rain erosion resistance. Really take a look at the Sensenich. You can always have a prop shop (or you) paint it some fancy color other than gray if you want style.
What was the deal with the RV-8?
You will love the RV-4, one of Van's best. Less room than a RV-8 yes but if you are not a high on the "Bubba scale" it fits fine. KEEP IT LIGHT!
PS: a lot of folks like the Carb I see above. I'm in the same Carb fan camp, for light and simple, but frankly buying a new engine today you have the choice between Carb and FI at the same base price. The FI installation is about a $700 more (high pressure boost pump), but FI is technically better once set up. FI fuel savings or potential savings, even if only 1% or 2% in cruise, means more now that gas is $4.50/gal. In the day most RV builders bought used engines and got what you could get, mostly carbs. FI is how I'd go today if buying a new engine, even if it cost a little more to buy and install.
360/180HP is pretty good trade off weight to HP. Cost diff is small between 320/360 if buying new. I would not go low compression 360, but it would be better for autogas. I'm not autogas fan. A 320/160HP have the same compression as a 360/180HP (8.5:1). 8:5:1 can get away with premium autogas I understand, but pretty soon all autogas will be ethanol infected. Yea I know ethanol is great?
Ethanol Rant: It takes almost more units of energy to make ethanol from corn than units of ethanol energy you get out. Ethanol has been over hyped by the Farm lobby.
DID YOU KNOW, price of BEER is going up!?! Why because Farmers are growing less lovely barley to grow more corn for ethanol. If that is not enough to hate ethanol, I don't know what is.
The reason ethanol works in Brazil is because they use sugar to make ethanol, which has a much more favorable energy in/out ratio than corn. Also I recall sugar uses less land than corn, per unit of energy. Corn is not a desirable crop to make ethanol and drives food cost up. We only use it in the USA because we grow lots of corn, and the Farmers in Iowa have a good political lobby.
