I don't know "a lot", but here's what I know:

I've only flown the RV-4, and from what I know about both types, the answer to your question depends on how, exactly, you define "performance".
They're very similar where it counts most: stall speed and takeoff/landing distances.
It's interesting to compare the two, because they can be built with the same engine.
Put a 160-hp engine in either plane, and you'll see roughly equal performance in climb and cruise.
The 8 is a bit roomier, carries more weight in people and baggage, and has more range, but the 4 will fly almost 800 miles at 170 mph while burning less than 8 gph.... and that's only with 150 hp. The one I have flown has  a 150-hp O-320, and it will cruise at over 140 mph at just shy of 6gph. That's the kind of performance that I like (because I'm not rich), and that's part of the reason why my friend owns that airplane.

I'd imagine that in terms of roll rate, etc. they are about the same (the 4's wing area is only 6 sq. ft. smaller, and the span is only a foot less than the 8).
On the other hand. the 8 will climb like a homesick angel with 200 hp available and can cruise very fast.

If the 8 is less sporty than the 4, it is probably still pretty damn sporty. The Rv-4 is an exciting airplane to fly- it rolls like a fighter and has excellent positive neutral stability (it tends to keep doing what you just told it to do). A little less of that would still be a lot of fun. 

But the RV-4 is not for husky folks who like to travel with a lot of stuff... hence the 8. Basically the same airplane, only with more room and a higher GTW, along with the ability to use a 200-hp engine.

If economy is an issue, go with the 4. If more X-C utility is needed, go with the 8.
But if all you want is raw excitement, buy or build a single-seat RV-3! 

OldReliable wrote on Nov 10^th, 2009 at 6:10pm:


That would be correct.  In aircraft, there is almost a direct relationship between horsepower and gallons per hour.  The nice thing about an RV-4, it doesn't take a whole lot of power to have fun.  We have one based here with only 125-135 horsepower, and while not the fastest RV around, still flies wonderful and is just fun.  He is only burning about 5 or 6 gph, so it is a cheap way to fly.  On the other hand we have a F-1 Rocket (www.teamrocketaircraft.com) here as well, which is an RV-4 on steroids, 260 hp.  He can cruise over 235 kts and climb like the name implies, but he is burning 10-12 gph.

There isn't a whole lot of baggage room in an RV-4.  If baggage is an issue, a -7 or -8 might make a better machine for you.  They both will give you the performance you are looking for, with a little bit more interior room.  The biggest issue with the tandem seat RVs is CoG, if you have too much weight in the back seat or baggage, it can make handling get a little tricky.  The -8 is better than the -4, since there is an extra baggage compartment in front of the cockpit, and -6 and -7 is even better, as the baggage is closer to the CoG.

OldReliable wrote on Nov 10^th, 2009 at 9:10pm:

You should take a look at the website... very comprehensive spec/performance/ FAQ coverage.

http://www.vansaircraft.com/

But to answer your question (assuming you mean Light Sport Aircraft): No. They're too fast. Top cruise speed for LSA-category birds is 120 kts. It might be possible to build one that only goes that fast... and meets the max. weight requirements... but it wouldn't be an RV-4/8 anymore, really.

But now there is the RV-12, which is an LSA... not quite as exciting as the others, but at least one could fly it under Sport rules, and it is a Van's...