Fourth pedal could be either, a "dead pedal" (somewhere to rest your foot when driving)
foot operated brake pedal, (mercedes have them)
or it could be a "line lock" pedal, to enable front brakes only to be applied, whilst winding up the motor for a quick take off. A lot of street racers use them..lets them lock the front and spin the back tyres for extra heat.
But a bit of googling later....it is an emergency brake pedal......basically a foot operated parking brake system.