The equation in that section for updating the concentration of C has a "4 * speed(t,x,y)" term, it seems like that should correspond to 4C in the reaction, especially because the equation for B has "-2 * speed(t,x,y)".
EDIT: Also, the text after the equations says "Notice that the numbers in front of speed(t) come from the quantities in the equation that summarizes the reaction."
EDIT: Also, the text after the equations says "Notice that the numbers in front of speed(t) come from the quantities in the equation that summarizes the reaction."