It is more likely 9. pH 4 is acidic and pH 9 is basic, and as the pH of a substance gets closer to 0 or 14, the substance becomes more corrosive or reactive. As 4 is closer to 0 than 9 is to 14, there is a much higher chance the solution has a pH of 9, because pH 4 is less neutral and therefore more corrosive/reactive than pH 9.
The oxidation half-reaction occurs at one electrode (the anode), and the reduction half-reaction occurs at the other (the cathode). When the circuit is closed, electrons flow from the anodeto the cathode.
