The reason you don't see many small 4 cyl bikes in the US is that they would cost too much. Would you pay 4,000 to 5,000 for one of those 400Fs? I'm sure some of us would, but not enough. Would it surprise you to know that only 3% of the US population owns motorcycles? That's street, off-road AND atvs. And when you couple that with the "bigger is better"attitude, we'll never get the cool small displacement bikes here.
Did you know that outside the US, they sell 250cc FOURS? They must wind up real good!