Look at classifieds of aircraft for sale. You will see the average RV-4 asking price is lower than an average RV-8. The lowest priced RV-4 will almost likely be much less than an RV-8. They are both RV's and thus fly like one... great. RV-4's are going to be older and may not have as modern panel, if not updated from the original build. RV-8's don't automatically mean a glass cockpit and they can be over 20 years old, but the introduction date of the kits are 17 years apart. Keep in mind some RV-4's are handmade works of arts and will command higher prices.
RV-4 is an amazing plane, kit released 1979. It was intended to utilize a 150-180 HP engine, fixed pitch prop, but many have put angle valve 200 HP Lyc and constant speed props on them. It has a tip over canopy.
The RV-8 came out 1996 is larger, still a two place tandem, the gear is taller (and different design), cockpit wider, more fuel, and it has a Fwd stowage compartment. It was made for a larger people, larger engine, including the angle valve 200HP Lyc and constant speed prop. Besides the sliding canopy.
Either is great, but the RV-8 is a "better plane", but a good example with the best goodies, glass panel, low time, nice paint, nice interior, might fetch $90,000 to $180,000 for good flying examples. RV-4's can be expensive but a typical one can go for much less than as an RV-8, as low as $40,000 ball park.
If you are BUILDING... RV-8 is a current kit. The RV-4 is also available. The RV-8 has pre punched parts that does not require you to drill the skins, ribs, spar, stringers, longerons. So RV-8 is the only way to go. The RV-4 has all the parts preformed and roughly cut to size like an RV-8 but the holes (mostly for rivets) must be drilled first. The RV-10 and RV-14 are even more advanced kits and easier still to build. No super human ability is needed to build an RV-4 or RV-8 for that matter but it will take time and effort.
BOTTOM LINE There is no bad RV.... they all are a delight to fly.... The RV-8 has more fuel, wider cockpit and easier to build kit and price of kit is irrelevant. The cost to build is driven by engine, prop, panel, electrical, paint, upholstery. The biggest of course is engine, prop and panel, but a custom paint job you sub out can be very expensive. These cost above the kit price are about the same for any RV. However a light RV-3 or RV-4 with a 150/160HP fixed wood prop, basic day VFR panel is a joy to fly. Many RV's get so weighed down with "stuff" they lose some of their feel and performance. If you never flew one it is shocking how nice it is.