4 valves are usually used on "small" engine to give them better power.
The down side of it is that without a variable intake/camshaft/valve, the extra power is on the top end.
What move a car,is torque and torque is directly proportional to size (liter/ci).
I agree that the horsepower per liter is not this good on the Viper,but with so many ci we don't need it.
Formula 1 engine crank out a lot of power from small engine,I think something like 800 hp from 1.5 liter but at very HIGH rpm,about 17,000rpm.
At the same horsepower rating,the car with the more torque will always be the car that is more fun/easier to drive and the less rpm,the longer the engine will last.
Ferrari is a good example,they use a high revving,high technology
engine that need a LOT of EXPENSIVE maintenance.
Also on a big engine,the weight of the reciprocating parts
(pistons,rods,etc)dictate a lower maximum rpm.
As the saying go; : "there is no replacement for displacement"
Luc.