Schyster is an automaker in Grand Theft Auto IV. It is based on Chrysler. The emblem appears to resemble a Plymouth badge, as Plymouth was a former brand marketed by Chrysler. The logo appears to feature a circular saw blade (maybe because the name looks somewhat like scythe or some other words pertaining to cutting). The name seems to be a reference to the word Shyster, which means a person acting unprofessionally.
For at least the duration of the Cabby's production, Schyster maintained a relationship with Vapid, which is based on Ford. The Cabby taxicab appears to be a re-badged Vapid Minivan and features a "Limited Vapid Design" badge.