There are several things to consider. In general a 4 will have a narrower spread of torque, but more power, rev higher and be smoother, but it depends on what the engine was designed to do. All 4s are not sports bikes, and all twins are not commuters.
Personally I find 4s lack character - and I've owned a few over the years - however much extra power they deliver. For smoothness a triple offers better balance, therefore potentially smoother, with an engine not much wider than a twin (but still a lot wider than a V twin) and an exhaust note that's different. 4s are smoother than twins, not really because of the extra power strokes, but because the 4 cylinder crank offers much better balance than the 360 twin. A 180 degree twin offers similar balance to the 4 but has a severe rocking couple and uneven firing interval. Balance shafts can only partially compensate for that. The triple, with perfect primary and secondary balance and an even firing interval still has the rocking couple, but because of the otherwise perfect balance it's easily offset by a small balance shaft.
V twin balance depends on the configuration of the V - the angle between the cylinders. It can be very good, but that's offset by the uneven firing interval. V twins probably have more character than any other engine, but character isn't something that's easy to measure.
I also find that bikes - and you have to look at the whole package, not just the engine - with 4 cylinder motors are less agile than twins. Sports 4s are more extreme, less comfortable and less suited to non-sports uses, but that only applies to sports 4s, and not to bikes intended for commuting or touring.
Middleweight twins are probably the best all rounders. You can't regularly do 150 mph in normal riding and expect to live, and you can't cut through congested traffic on a 600 lb lump of cruiser or on a sports bike where you drag your knuckles on the road reaching down for the handlebars.
Last point. Engines dont make power. They only make torque. Power is the expression of the work done by the application of that torque. 4s do make more torque than twins at the wheel (though often less at the crank) - it's just further up the engine rev band and develops over a narrower spread of revs.
There's an often misquoted equation that says power = torque x revs. Thats an over simplification, but it is basically true. What people forget though, is when you're using a dyno it's the torque at the wheel (the gearbox is a torque multiplier) and the revs that the wheel, not the engine, is making. Torque and revs at the crank are back calculated for the chart and the results are distorted by the characteristics of the dyno and by transmission losses.
Last edited by williamr; 05-25-2010 at 09:22 AM.