You are mistaken, itachi needs level 25. What I think you misinterpreting is where it says "Assist links required: 1". That means there's only ninja that needs to be attached to itachi, in this case shisui. This is why you would think Orochimaru needs level 4, since what he needs is all four Sound Four ninjas to activate. In both cases, as for all bond mysteries, you still need to the have total level across all bond cards attached to that be 25 as well as have the appropriate cards attached. This is why no one really uses orochimaru bond skill since you HAVE to level up the sound four cards, an investment that no one really wants to make.