"Good", to me, means to do something that brings out the most beneficial outcome for all involved parties.
For instance, being a "good person" would include helping others or showing understanding for their issues and problems whilst trying to find a good solution that all can agree too (in case of a fight for example). I think I treasure positive emotional values over the physical ones however. Like, giving someone a present is far lower in its "goodness" measures, than for instance, emotional support. The only physical exception is purchasing things for someone in need of such thing, such as food or clothes or other, practical and necessary objects.
Am I a good person? Naw. I have enough negative traits and anger issues to consider myself as just an average person. I think, there are not really any "good" people as in perfect "goodness". Even those who are selfless, while technically good to others, often times forget themselves and treat themselves badly. This isn't always the case, but I have seen it happen.
I'd rather put myself as an "kinda okay" person, because while I do strive to be nice and polite and well, "good" to people, my flaws just sometimes overweigh that.
So, question number four comes down to this: "Good" exists, but in its purest form it's probably just a theoretical idea. Everyone who tries to be good has something about it in them however.
I also want to mention there is "doing what you think is good" showing that there is another part to this entirely. Everyone views what is good in a different way. Some would say that , for example, it's good to reduce how much meat we eat and thus make cow farms less, in order to fight against the way cows are held and support a healthier lifestyle, while others think it's good to support cow farms for jobs, meat and of course tasty hamburgers and say that you can eat beef and still be perfectly healthy.