As per my experiance, different type of plugs dont make much of difference as long as gap and heat range is as per specifications. The spark plugs only initiate the spark and dosnt effect the flame speed etc. Flame speed is function of air/fuel mixture properties. If fast flame fasr piston theory is presumed correct, we will have to change ignition timing with different type of plugs which is not the case. Also in this case we will have different idle rpms for different types of plugs which is again not the case.
Moreover, any plug can only burn the fuel available in combustion chamber and not any more.
Super 4 plugs r required with modern high compression ratio and/or lean burn type engines which incorporate hegh energy ignitions.
So my recommendation is to stick with stock plugs and just reduce gap to .6 mm for operation on CNG