That "residual spark" theory is a red herring.
When #1 fires on compression, #4 fires on exhuast as the Bob says but that's a consequence of the 8 plugs sharing 4 coils.
To be able to control 8 plugs individually would otherwise require 8 coils (Scaramanga!) so instead the "exhaust" spark is a wasted one.
If the ignition system is doing its thing properly, there should definitely not be anything there for the "on exhaust" plug firing to burn off.
The extra plug is designed to trap any mixture that's lurking around the outer edges of the piston - more important on those high-compression, carburetted race engines of the sixties (where the pistons had a small mountain range cast into their crowns to boost compression) than on a state-of-the-art injected road-going engine, but what the hell. It's good for the brand image, innit?