We all know a four does not sound like an eight, but why doesn’t a four cylinder engine at 4000 rpm sound like a V8 at 2000 rpm. (Both engines should be firing the same number of detonations per minute)
I doubt detonations per minute is the main factor in how an engine sounds. Displacement is probably bigger factor.
Most Harleys have what, 2 cylinders? My Goldwing has 6 cylinders. Sound doesn’t appear to be related to number of cylinders.
Lots of variables affect engine sound – number of cylinders, exhaust system, firing order, cylinder angle, displacement, turbo/super chargers, etc.
Twotone
Displacement, piston travel, valves, and intake/exhaust sound modifiers.
Want your 4-banger to sound more powerful for free? Remove the intake resonator.
Want it to sound like a race car? Remove everything on the exhaust behind the catalytic converter. The V8 guy will be amazed. And then he’ll go deaf.
Blip The Throttle On My Old (Former) 4 Cylinder VW Dune Buggy With A Holley Bug-Spray Carb, 1700 CC Jugs, And Tee-Pee Exhaust (With No Muffler) And The Sound Said, " Get Your Kids Off The Street ! "
Maybe horizontally opposed was part of the fomula. Headers, exhaust pipe length or exhaust arrangement could be another.
CSA
Why Did The Old 650 Triumph Bonnevilles (360 ?) Sound Like Music To My Ears (Even Without Muffler) ? Could It Be That Some 2 Cylinder Motorcycles Have 360 Degree Cranks And Some 180 Degree ? I Think So.
When I was a kid my 64 Honda 305 Super-Hawk (180 ?) sounded like all Hell was breaking loose (not very melodic) when I’d remove the mufflers and cruise the neighborhood. A cold day was like adding a super-charger to it.
CSA
American V-8s usually 90 degree crank throws which allows a smoother firing order, but results in the exhaust exiting the manifolds in uneven intervals. Listen to a Ferrari V-8 with a “flat plane” crank shaft. They have a very distinctive howl that is nothing like the rumble that american V-8s produce.
I doubt detonations per minute is the main factor in how an engine sounds
In the 4 banger, it’s the difference between sounding like the PH in “my PHat ride” and an angry insect… 
This is a very good explanation for the syncopated sound of a 90 degree crankshaft V8 that otherwise has an even firing order. When Cadillac released their first V8 in 1915, it had a flat plane crank similar to many WW1 airplane engines. Manufacturing techniques advanced to where Cadillac could release a 90 degree V8 crank in 1923 which then made the rumble sound but reduced vibration. A flat plane crank such as what Ferrari and some other racing cars use does not require counterweights which makes it lighter to enhance acceleration.