Sorry if that sounded a bit heavy handed, what I should have said was WE don't know the cost of the RSV4's motor (unless someone here works for Piaggio).
Here's what it comes down to:
Horsepower is all about pumping lots and lots of air. Big pistons are limited (due to their mass) in how fast they can go back and forth. So you use smaller pistons, and spin the engine at high speed.
All else equal (displacement, stroke, etc) inline fours and V4s have the exact same size pistons. Theoretical horsepower between the two engine layouts is basically the same.
In MotoGP, the choice of engine layout comes down to things like width, weight distribution, and moment of inertia advantages/disadvantages of the different layouts. In that world, choosing the engine configuration is not about cost or horsepower, it's about handling and packaging.
But for production bikes, which is a much different world, it's purely down to cost. At this point any of the major OEMs could make 220 horsepower production engines in either inline or V4 configurations, if they chose to do so. It's cheaper to do it as an inline, though.
Ever ask yourself why anyone makes parallel twins instead of v twins? Exact same reason and principle as inline vs. V4, just applied to an engine with two less cylinders.
