I think that you need to think about what types of patterns you are setting up for your child. Are you going to have her grow up with you banning anything with the potential for a conversation?
At 4 years old, she cannot possibly understand what you are so upset about. I am sure she sees kissing between you and your spouse, her brother and sister, other relitives and friends, and depending on your house rules, possibly television. How is she to comprehend at 4 years old that this is something that is sexually based, and not an act of love that one person shows another?
You need to have a conversation with her, at her level, if this is something that you do not want to continue. You have to understand that as a 4 year old, she will probably defy you. If she continues kissing friends, are you going to ban all of them from your home? Come up with a gentle way to explain that there are different types of kissing, and which are and are not appropriate.
I believe this will do you a lot better than setting up a pattern for banning at any slight infraction.
With all of the conversation this has caused on this message board, I am curious to hear how you have decided to handle this.
The best of luck to you,