With respect to #4, an example might clear it up:
/C –> /V. (If you’re not a citizen, you cannot vote)
V –>. C. (If you can vote, you’re a citizen).
It works the same way it does in the example where nothing is negated. V must guarantee C, because (original statement) if we didn’t have C, we couldn’t have V.
For #2, consider a game show where the winner can choose one (but only one) of two prizes:
/C –> V. (If you don’t take the car, you can have the vacation).
/V –> C. (If you don’t take the vacation, you can have the car).
/V guarantees C, because if we didn’t have C, we could have V.*
^ In this example, C and V would be “eligibility for the car/vacation”.
Hope this helps
-Dan