when you have your 4 different equations to form 4 different possible equations, what two equations do you use for water,
O2(aq) + 4H+ + 4e- ----> 2H20 (+1.23)
02 (g) + 2H2O (l) + 4e- ----> 4OH- (+0.40)
2H20 (l) + 2e- ---> h2 (g) + 2OH- (aq) (-0.83)
I'm not quite sure what you are after but I need to point out that you make a statement about four different equations but write only three. Then you ask what two equations to use (out of the three or out of four)? Anyway, the first one occurs in acid solution and the last two occur in basic solution.
To choose the two equations related to water, you should consider the conditions under which you are studying the reaction.
If you are studying the reaction in an acidic solution, you would use the first equation since it occurs in acid solution and involves both water and oxygen.
If you are studying the reaction in a basic solution, you would use the second and third equations. The second equation shows the reduction of oxygen in a basic solution with the production of hydroxide ions. The third equation shows the reduction of water to produce hydrogen gas and hydroxide ions.
So, to summarize:
For an acidic solution: O2(aq) + 4H+ + 4e- ----> 2H20 (+1.23)
For a basic solution:
Equation 1: O2 (g) + 2H2O (l) + 4e- ----> 4OH- (+0.40)
Equation 2: 2H20 (l) + 2e- ---> H2 (g) + 2OH- (aq) (-0.83)
Remember, the choice of equations depends on the conditions of your experiment or the context of the problem you are trying to solve.