Part warm body, part social chameleon, fourths have become an accepted part of the commuting landscape.
This is instantly intriguing. It works because I believe the premise. It works because it's not the endless stream of same-old set up. (Demons, vampires, girls who must come to terms with being outsiders cause they're were-cheerleaders)
I always like to edit out "Since" as the way to start a sentence. Use "since" sparingly since it requires a "must" clause, and you end up with long-ass sentences.
It's an easy way to earn some extra cash--or to end up dead. Someone is killing fourths and the only one who seems to care is burnt-out homicide cop Francis LaCroix, who moonlights as a fourth himself.
This neatly solves the problem of stakes (the cop might get killed) and why the protagonist needs to investigate (he's a cop, and he's a potential victim.)
And there's the twist that gets me really interested.
TAKING THE HIGHWAY,
This isn't science fiction. And I'd STRONGLY urge you to not call it science fiction even if you think it is. There's a lot of room for cross-over into crime fiction here, and by calling it science fiction you might miss an agent who doesn't handle SF but would read this. Like...me.
Thank you for your consideration.