Ethics in putting my co-supervisor's name in a conference talk: his name did not appear in the book of abstracts
I would think that most people would understand if you explain it just as you did here. You listed four names and the conference removed the fourth. I think it is common, even reasonable, for your supervisor to want to advance the interests of your co-supervisor, but it is too late for any correction. 

However, in the future you would be wise to check such things with everyone before you submit anything, especially when the conference or journal imposes rules that you think are not optimal. I would then try very hard to follow the advice given. 

The fact that the co-supervisor didn't contribute shouldn't be raised in your meeting as there is nothing to gain from it - for anyone. While in the best of all possible worlds it should mean something, you can only lose by seeming to speak against your supervisor at this moment. 

Whether your supervisor and/or co-supervisor is behaving well or not is also not a consideration for you at this point. If you can't make a change in the situation, work on the relationships so that everyone can move forward and work effectively. In poker terms, play the hand you are dealt, not the hand you wished for.