The 8 plug are a benefit too, the 4 main plugs (14mm) are the main firing plugs which ignite the mix at the optimum time in the cycle. The other 4 plugs (10mm) are there to speed up the flow of gases through the engine by igniting unburnt mixture in the cylinders, they also add about 5Bhp as a byproduct.
Pretty dubious about this, twin spark makes sense in a two-valve per cylinder head or a 3-valve but not in a 4-valve head as the spark plug is ideally located centrally in the space between the four valves. One of the limiting factors of any engine is flame speed from the spark plug to the end-gases at the outer edge of the piston. This speed remains revatively constant thoughout the rev range, i.e. the time for the fuel air mix to burn is pretty much the same at 2000 rpm as it is at 7000. This is why the ignition point is advanced earlier in the compression stroke as the rpm goes up. Lighting the mix earlier ensures that all the fuel is burnt and maximum pressure reached just after TDC on the power stroke.
In the 8 valve twinnie, (and in 3 valve per cylinder twin sparks like some Merc engines) each plugs sat on opposite sides of the combustion chamber. With a single spark plug the flame has to travel from almost one side of the chamber to the other. A plug on the otherside firing at the same time halves the time for the burn by lighting the mix at either end of the chamber. The original Alfa 8 valve twin spark engine ran very little spark advance in comparison to its single plug comptemporaries.
No such advantage exists in four valve head as the burn path from the centre plug is equi-distant from the cylinder walls. There may even be a disadvantage in a second side mounted plug in a 4 valve head which occupies what in most 4 valve heads is a valuable squish/quench zone, that either aids the burn by creating turbulence, and/or stablises the unburnt end gases the small plug is supposed to ignite... well, at least on one side of the cylinder.
The Twin Spark 16 valve is really for marketing purposes only. It was a real performance advantage like having 4 valves rather than 2 or 3, multi point fuel-injection, etc, everyone would be using them.
The wasted spark is no special feature either, every Japanese two and four cylinder motorcycles since Adam was a Cowboy have been wasted spark. As this is a simpler and therefore cheaper way of operating mutliple coil systems -only two triggers/pick ups/points instead of four. Most 4-cylinder bikes only have two coils each with two leads, one that fires the plugs in cylinders 1 and 4 simulatanously, the other 2 and 3.
One of each pair of cylinders will be on the end of its compression stroke and will ignite on the plug firing, the other cylinder is at the end of its exhaust stroke. Its called a 'wasted spark' because firing a plug in a cylinder that has pretty much blow down at the end of its exhaust stroke doesn't do anything. The 16 valve twinnie is a lean burn engine, so by the time the wasted spark fires there isn't anything left in the combustion chamber to burn, let alone give you 5bhp.