This is because you have strayed from the path. The Mk. III is the one true God.I've been flying my Viper Mk4 recently and, I gotta say, the noise it makes is badly wasted on such a mediocre ship.
It sounds like there's an angry tiger loose in the cargo hold.
I mean really - there is basically no point to the Mk. IV. It's like a Mk. III, but with a bit more cargo and a whole lot more crappiness. If I wanted to fly a bathtub, I'd buy a bathtub.