Mah Nishtanah is not really four questions at all. It is an exclamation - “How different this night is from all other nights!” - with four examples.
The first three examples, dealing with matzah, bitter herbs and dipping, come from the Mishnah Pesachim. The fourth was originally about why it was roast meat that was eaten at the Seder, but after the Temple was destroyed and the sacrifices were suspended this was replaced by the question about sitting or leaning.
There is no question about wine, both because this is not unique to Pesach and it is not specifically commanded in the Torah, like matzah and maror are.Placing Mah Nishtanah at the beginning of the Seder is somewhat illogical: the questioner has not yet tasted or possibly even seen the things he or she is querying!
Originally, however, Mah Nishtanah followed the meal, and then the sages shifted it to its present position in order to provide a peg on which to hang the narration of the story and to ensure that the children would be awake to hear it.
If no children are present, the youngest person present asks the questions. If a married couple are having Seder on their own, the wife asks her husband. A person celebrating Seder without any company asks the questions of him- or herself. (An additional question in such circumstances is why they did not invite other people to share the occasion, since hospitality is not only good for guests but for the host too.)
Are there any answers to the four questions?
It does not appear so. All we get is Avadim Hayinu, “We were slaves to Pharaoh in Egypt”, and the narration of the events that led to the Exodus and freedom and unfolded the future. Yes, eventually there is Rabban Gamliel’s explanation of matzah and maror, but there are no answers to the questions about dipping or leaning.
Indeed, perhaps even Rabban Gamliel himself is not really answering the questions. All he is doing is taking part in the story, which shows that if you know the circumstances and understand the background, the questions become largely redundant.