I think the best course of action is probably to be completely honest. Sit down with everyone, ask them not to interrupt you but to hear everything you have to say, and explain that yes, you kissed a guy recently and yes, you're dating one. Tell them that you felt so hurt, rejected and guilty because of their reaction four years ago that you lied to make them feel better. Say that you've realised that you can't live that lie because it would make you deeply unhappy, that your boyfriend does make you happy, and that you need your family to accept and love you for you are, because you're still the same person you've always been. Ask them to think about what you've said before reacting, tell them that you love them, and that you hope they'll still love you too.
Hopefully that will remind them that it is their duty as your family to love and accept you despite your sexuality, that it doesn't matter in the scheme of things or change who you are, and make them remember that they do want you to be happy. Basically the idea is to pull on their heartstrings and make them realise how terrible it would be of them to react badly again.
All the best!