There are entire books that cover why a 4 cylinder will always make more power than a twin, at a given displacement. It basically, however, has to do with its ability to rev.
A twin has a bigger bore and stroke, assume all the motors have piston speeds of around 4000feet per minute. that is alot. twins have a longer stroke, so they carry less revs at that 4000fpm ceiling. HP is something like torque*rpm/5225 i cant remember exactly and dont feel like looking it up. so anyways, that means a higher revving inline 4 will be able to use its higher rpms at a given displacement to make more HP. This is also the reason for the difference in displacement.
However, Twins have an advantage in other areas. Their firing order puts 2 pulses right next together, so there are little pauses in its power. this allows the tire to recover and get good drive. Inline 4's tend to have even firing spacing, either 180 or 360 degrees. this means a cylinder is always firing, and this is supposedly why the twins have better "drive" off of corners. When they talk about "torque" being the advantage, i think what they mean is that the 4 cylinders make the same power higher up, so the tire will want to sllide sooner. Its not that they need more torque, its just that they need it lower, so there is more time between firing of the pistons, again, for grip.
Thats the theory of it, anyways. The real bread and butter comes down to money, i think, although Ducati has someone debunked that, as they are giving Honda a run. It could just be because Ducati is much more of a racing oriented company, and that makes up for any financial shortcomings.
And also, those Kawasakis are friggin tanks!!! those frames really are dented to *****, and they are still running them!